TOOLS-4263 Convert mongorestore bulk sizing tests to Go

[autarch]

Adds integration/dumprestore/bulk_test.go: two DumpRestoreSuite methods, five cases in total, covering how mongorestore batches documents when writing them back.

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

• test/qa-tests/jstests/restore/large_bulk.js -> TestRestoreLargeBulk (integration/dumprestore/bulk_test.go) - multi-megabyte documents survive a dump and restore byte for byte, batched together rather than written one at a time. Worth recording what this test does not cover: the JS was added for TOOLS-939, where mongorestore batched by document count alone and could build a write command larger than the server would accept, but it never actually reached that threshold. The buffered bulk inserter flushes at MAX_MESSAGE_SIZE_BYTES minus 1MB, roughly 47MB (common/db/buffered_bulk.go), not at the 16MB MaxBSONSize that limits a single document, and the JS fixture totaled only about 35MB. Growing the fixture past 47MB would make every CI topology and server version pay for it, and the size-based flush is already covered by unit tests in common/db/buffered_bulk_test.go (the "byte limit 1" and "full buffer with max-length namespace" cases), so this test keeps only the assertion it can honestly make.
• test/qa-tests/jstests/restore/duplicate_keys.js -> TestRestoreDuplicateKeys (integration/dumprestore/bulk_test.go) - restoring a dump over a collection that still holds most of the dumped documents inserts the missing ones while the rest produce duplicate key errors, which mongorestore reports as failures without failing the restore. This is the first coverage of --batchSize. The JS asserted only the exit code and the final document count, which cannot tell "the ten missing documents were reinserted" apart from "all fifty were overwritten"; this marks the surviving documents after the dump is taken, so the marker exists nowhere in the dump and would disappear if a document were overwritten, and it asserts the exact split of inserts to duplicate-key failures. The JS looped --batchSize from 1 to 99, which is 99 dump and restore cycles for one behavior; this uses the default, 1, 7 so that the final batch is partial rather than full, and 51 so that every document lands in a single batch.

[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 👈 (View in Graphite)
• 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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