internal/testing/setup: close the fake credentials file

[TuSKan]

FakeGCPDefaultCredentials creates the file with os.CreateTemp and never closes the returned handle. On Windows an open handle blocks deletion, so t.TempDir's cleanup fails and the test reports:

TempDir RemoveAll cleanup: unlinkat ...\fake-gcp-creds...:
The process cannot access the file because it is being used by another process.

That is enough to fail a package on Windows even when the test itself passes. On stock master:

$ go test ./gcp/ -run Credentials
--- FAIL: TestDefaultCredentialsWithParams
    testing.go:1464: TempDir RemoveAll cleanup: unlinkat ...: The process
    cannot access the file because it is being used by another process.
FAIL

and with this change:

$ go test ./gcp/ -run Credentials
ok      gocloud.dev/gcp

The handle is now closed before the file is written to by name. Nine other packages call FakeGCPDefaultCredentials and are affected the same way.

Verified with go build ./..., gofmt -s, go vet, and golangci-lint run (0 issues).

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.42%. Comparing base (b5401c0) to head (8843fbd).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/testing/setup/setup.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3768   +/-   ##
=======================================
  Coverage   75.41%   75.42%           
=======================================
  Files         104      104           
  Lines       14262    14267    +5     
=======================================
+ Hits        10756    10761    +5     
+ Misses       2769     2768    -1     
- Partials      737      738    +1     

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