Remove hidden unit test dependency on GCR creds

[imjasonh]

By having pkg/crane/copy_test.go set up its fake registry at gcr.io, this seems to trigger behavior that ends up looking for default GCP credentials to authorize pushes/pulls. This works fine when tests run in GCB, and presumably on most of our local workstations, where GCP creds happen to be available, but fails on GitHub Actions, where creds are not available.

Instead of calling our fake registry "gcr.io", this change renames it to "xcr.io".

This change includes the Unit Tests action being added by #792, to check that this change fixes the unit test failure.

[imjasonh]

cc @mattmoor

[codecov-io]

Codecov Report

Merging #793 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #793   +/-   ##
=======================================
  Coverage   75.54%   75.54%           
=======================================
  Files         102      102           
  Lines        4171     4171           
=======================================
  Hits         3151     3151           
  Misses        566      566           
  Partials      454      454           

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