src/goTest: Add command Test Current Package With Custom Run

[n1lesh]

This runs Test Current Package for a custom value passed for the -run flag.

Fixes #1734

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This PR (HEAD: 581c7fb) has been imported to Gerrit for code review.

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Message from Ethan Reesor:

Patch Set 3:

(1 comment)

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This PR (HEAD: 2432329) has been imported to Gerrit for code review.

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Message from Nilesh Singh:

Patch Set 3:

(3 comments)

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[n1lesh]

@firelizzard18 Would it be possible for you to review this?

[firelizzard18]

@n1lesh I reviewed the CL, but I'm not part of the Go team so I can't do anything other than make suggestions. AFAIK, @hyangah is responsible for reviewing CLs on this repo.