[go_tests] Deduplicate runs per module

[duckhawk]

Summary

The go_tests action previously iterated over every *_test.go file under images/ and ran go test in the enclosing directory without specifying a package or file. For a package with N test files and M build tags, this produced N*M runs of the same package instead of M.

This PR switches the outer loop to iterate over go.mod files (one per Go module) and runs go test ./... -tags <tag> once per module per build tag. The pattern matches what is already used in the sibling actions:

• go_linter
• go_modules_check
• go_test_coverage

Additional small improvements:

• restore the working directory between modules (use cd "$basedir" instead of letting it drift),
• use dirname instead of a fragile sed regex on the file name,
• quote variables and use [ "$failed" = 'true' ] for portability.

The same fix has already been applied to the v12 branch.

Test plan

• Trigger the go_tests action in a downstream module repository.
• Confirm in the job log that each Go module is tested once per build tag (no more duplicate runs per test file).
• Verify failing tests still cause the action to exit with non-zero status.