Fix existing golangci-lint warnings in tests

[corylanou]

Problem

A full golangci-lint run currently reports five pre-existing warnings in test code. These were surfaced while reviewing #97 but are unrelated to that PR and should be fixed separately.

Unchecked file-close errors (errcheck)

• checksum_test.go:83: defer f.Close() in testChecksumPages
• checksum_test.go:116: defer f.Close() in legacyChecksumPages

Handle the close results explicitly using the repository test/helper conventions. Preserve any earlier operation error when propagating a close error from legacyChecksumPages.

Deprecated random reads (staticcheck SA1019)

• ltx_test.go:698: rand.Read(data) in benchmarkChecksumPage
• ltx_test.go:718: rand.Read(data) in benchmarkChecksumPageWithHasher
• ltx_test.go:737: rand.Read(page) in BenchmarkXOR

math/rand.Read is deprecated. Replace it with an appropriate deterministic math/rand/v2 source for benchmark fixture data, or use crypto/rand if nondeterministic data is intentionally required. Keep random-data setup outside the timed benchmark sections.

Acceptance criteria

• golangci-lint run exits successfully with no findings.
• go test ./... passes.
• go test -race ./... passes.
• Existing benchmarks continue to operate on populated page data without adding random-data generation to their timed sections.
• No production behavior changes are included.