TOOLS-4263 Convert mongorestore drop, path and stdin tests to Go

[autarch]

Adds integration/dumprestore/drop_test.go and integration/dumprestore/paths_test.go, extends TestPipedDumpRestore and TestRestoreInvalidInput, and deletes the seven JS tests they replace plus the symlink fixture that only one of them used.

JS -> Go mapping:

• test/qa-tests/jstests/restore/drop_with_data.js -> TestRestoreDropWithData (integration/dumprestore/drop_test.go) - --drop replaces the existing contents of two collections. The pre-dump data is swapped for a disjoint set of documents with a different count before the restore, so a restore that upserted by _id rather than dropping leaves both sets behind and fails on content, not just on a count that happens to match.
• test/qa-tests/jstests/restore/drop_one_collection.js -> TestRestoreDropOneCollection (integration/dumprestore/drop_test.go) - --drop scoped with --collection replaces only that collection, leaving its sibling and a second database holding the same collection names untouched.
• test/qa-tests/jstests/restore/drop_nonexistent_db.js -> TestRestoreDropNonexistentDB (integration/dumprestore/drop_test.go) - --drop against a database that does not exist is not an error and restores normally.
• test/qa-tests/jstests/restore/slash_in_collectionname.js -> TestRestoreSpecialCollectionNames (integration/dumprestore/paths_test.go) - collections named coll/foo, coll%bar and coll%2Fbaz round-trip through a directory-format dump, where the name has to survive being escaped into a filename and unescaped again. The third name is a double-escaping guard: if % ever stopped being escaped on the dump side, coll%2Fbaz would unescape back to coll/baz and restore into the wrong namespace.
• test/qa-tests/jstests/restore/archive_stdout.js -> TestArchiveThroughStdout (integration/dumprestore/paths_test.go) - a real mongodump --archive process has its standard output piped into a real mongorestore --archive --drop process. TestPipedDumpRestore covers the archive format but connects the two in-process through an io.Pipe, so the CLI plumbing here (--archive with no value selecting stdout and stdin, and a binary archive surviving a real pipe) is not covered anywhere else.
• test/qa-tests/jstests/restore/symlinks.js -> TestRestoreDumpWithSymlinks (integration/dumprestore/paths_test.go) - a dump directory in which a database directory and a collection's bson file are both symlinks restores correctly, and a symlink to a plain file at the top level is not walked as a database directory.
• test/legacy42/jstests/tool/dumprestore1.js -> three subtests in TestRestoreInvalidInput (mongorestore/invalid_input_test.go) - a restore target of - reads from stdin, which carries no namespace of its own, so it is rejected with only --db, rejected with only --collection, and accepted with both. The accepting case matters: without it the two rejections would pass even if --dir - were rejected unconditionally.

Two things worth flagging for review.

The symlink fixture has no symlinks. Every entry under test/qa-tests/jstests/restore/testdata/dump_with_soft_links/ is a regular file (mode 100644), and the supposed link targets are duplicated blobs: dbOne/data.bson and soft_linked_collection.bson are the same object, as are dbTwo/data.bson and soft_linked_db/data.bson. The links were flattened into copies at some point, so symlinks.js has been exercising an ordinary dump directory. The Go test builds its dump with real symlinks at runtime, which makes it a coverage increase rather than a like-for-like port, and the fixture is deleted rather than carried forward.

TestPipedDumpRestore now verifies what it restored. It previously renamed every namespace and then asserted only that Restore() returned no error, never reading back a document, so a namespace dropped or misrouted by the archive demultiplexer would have gone unnoticed. It now checks every destination namespace.

[autarch]

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• TOOLS-4263 Convert mongodump oplog consistency tests to Go #1104
• TOOLS-4263 Convert mongodump query and data behavior tests to Go #1102
• TOOLS-4263 Convert mongodump flag validation tests to Go #1098
• TOOLS-4263 Convert mongorestore users and roles temp collection tests to Go #1097
• TOOLS-4263 Convert mongorestore drop, path and stdin tests to Go #1096 👈 (View in Graphite)
• TOOLS-4263 Convert mongorestore namespace mapping tests to Go #1093
• TOOLS-4263 Convert mongorestore bulk sizing tests to Go #1091
• TOOLS-4263 Convert mongorestore index round-trip tests to Go #1074
• TOOLS-4263 Convert mongorestore collection-options and metadata tests to Go #1073
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