Fix pgTAP single-file working dir

[Flamki]

Fixes #4850

When running a single pgTAP test file, pg_prove was invoked with only the file bind-mounted.
This meant psql \ir includes (relative to the file) failed because the directory wasn’t mounted
and working dir didn’t resolve correctly.

Change:

• Mount the test file’s directory (or the test directory itself) into the container
• Set WorkingDir to that directory
• De-duplicate/sort binds

[coveralls]

Pull Request Test Coverage Report for Build 21874613363

Details

• 14 of 17 (82.35%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.003%) to 60.576%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/db/test/test.go	14	17	82.35%

Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 21859441939:	0.003%
Covered Lines:	7761
Relevant Lines:	12812

---

💛 - Coveralls

[Flamki]

CI status note:

• CI / Codegen fails during go generate with DiskRequestBody discriminator mapping (pre-existing issue, unrelated to this change).
• CI / Link fails due to missing project-ref flag in CI.

All build/lint/test checks for this change are green.