Default to previously set coverage file

[dsvensson]

Subject says it all, when running tests with coverage, don't have the user browse around for the coverage report file every time.

[freeekanayaka]

Or perhaps more simply (implementation-wise), add a custom variable (e.g. go-test-coverage-file) which is an empty string by default, but, if set, makes go-test-current-coverage use that filename without prompting it.

[freeekanayaka]

Another low-hanging option would be to let go-test-current-coverage accept an optional parameter (the coverage file).