feat: add ko test command to build with go test -c

[vincent-d]

Add a new command to build images with the output of go test -c.

It allows configuring build and LD flags in .ko.yaml with different values for build and test commands for a given importpath.

This is a new attempt at implementing #96 with a separate command.

[cpanato]

small comments for now

[github-actions]

This Pull Request is stale because it has been open for 90 days with
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[StevenACoffman]

Oh noes! This could be helpful!

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[clementnuss]

what is missing in this PR?

not being able to compile test binaries with ko is a blocker. hope someone finds the time to merge this 🙏🏼

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[mkilchhofer]

@vincent-d @cpanato can we reopen this?

[mkilchhofer]

Another friendly ping @vincent-d @cpanato @imjasonh - can we reopen this?