fix(gcsfs): honor TestMain exit code from m.Run

[00200200]

Summary

• defer os.Exit(exitCode) in gcsfs TestMain evaluated exitCode at defer registration time (always 0), so failed tests and panic recovery still exited successfully.
• Exit via defer func() { os.Exit(exitCode) }() so the final value from m.Run() / recover is used and CI reports failures correctly.

Fixes #681

Test plan

• Before fix: TestGcsReaddir / TestGcsReaddirnames / TestGcsGlob fail in output but go test exits 0
• After fix: same failures exit 1
• Passing subset (TestGcsFsOpen|Read|Seek|Name|Close|FsStat|Mkdir|UploadChunkSize) still exits 0
• Note: this intentionally surfaces the existing three mocked-directory failures; fixing those is out of scope here (see also Map the empty path to the root like "." and ".." #679)

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[00200200]

Closing this PR as I will not be signing the CLA. Thank you!