Rename metrics_test_utils.go to avoid dependency on testing package.

[ascherkus]

Any code depending on the exporter would pull in the testing package (noticable
when listing flags).

Rename file to metrics_utils_test.go to limit inclusion to test binaries.
Verified by running go list -deps . | grep testing.

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[ascherkus]

Didn't see the issue but this does indeed fix #229

[rghetia]

@ascherkus can you please sign the CLA?

[ascherkus]

@googlebot I signed it!

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