TOOLS-4263 Convert mongorestore index round-trip tests to Go

[autarch]

Adds integration/dumprestore/index_roundtrip_test.go: covering how mongorestore round-trips index specifications.

JS -> Go mapping (all Go tests live in integration/dumprestore/index_roundtrip_test.go):

• test/qa-tests/jstests/restore/indexes.js -> TestIndexRoundTrip (integration/dumprestore/index_roundtrip_test.go) - one collection carrying every index type the JS covered (simple, sparse and unique, compound, compound with int64 key values, multikey via array data, text with a non-default default_language, and 2dsphere) survives a dump/restore round trip, restored both from the dump directory and from the BSON file directly. The JS only compared index counts, so a mangled text-index weight or a dropped sparse flag would have gone unnoticed; this compares the complete spec of every index before and after, with fixture preconditions asserting the int64 key types and the Spanish text language really landed so that we're testing what we think we're testing. Note that the JS passed {language: 'spanish'}, which is not the createIndex option name and was therefore ignored by the server, so the Spanish text index is new coverage.
• test/legacy42/jstests/tool/dumprestore6.js -> TestRestoreLegacySystemIndexes (integration/dumprestore/index_roundtrip_test.go) - a pre-2.6 dump stores index specs in a system.indexes.bson file rather than a per-collection metadata file, and mongorestore falls back to that file when a database directory has no metadata. The test builds such a dump at runtime, deliberately naming a different database in the specs' ns fields than the restore target so that mongorestore has to take the target from --db, then asserts the document and both indexes are created and the legacy v1 specs are converted to the current index version.
• test/qa-tests/jstests/restore/index_version_roundtrip.js -> TestIndexVersionRoundTrip (integration/dumprestore/index_roundtrip_test.go) - --keepIndexVersion restores each _id index at its original version, while a restore without the flag gets the server's default version instead. Coverage is limited to _id indexes because the version can only be set through the create command's idIndex argument: a v1 secondary index is silently upgraded to the current version at creation on supported servers, so there is no secondary index version to round-trip. The secondary indexes are still created and asserted to survive both restores, along with the document counts.
• test/qa-tests/jstests/restore/ordered_partial_index.js -> TestOrderedPartialIndex (integration/dumprestore/index_roundtrip_test.go) - an index whose partialFilterExpression has ten fields keeps that document's field order through the metadata file, since reordering it would produce an index the server treats as different from the one that was dumped. The fields are written in a deliberately unsorted order and the restored spec is read back as a bson.D, so both a sorted reordering and a randomized one are detectable; an ascending field list would have matched its own most likely corruption.

[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
• 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 👈 (View in Graphite)
• TOOLS-4263 Convert mongorestore collection-options and metadata tests to Go #1073
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