Fix scripts tests

[philwalk]

• Verify return value of call to which("scalac") before attempting test. Same for which("scala").
• added execute permission to compiler/test-resources/scripting/showArgs.sc

This test may be brittle in that it assumes that the target scalac is in the PATH, or at least that no other version is in the PATH.

Fixes #12962

[dwijnand]

Rather than using which, could we switch this to dist/bin/scalac and dist/bin/scala and let the test fail if those aren't there or aren't executable?

[philwalk]

Rather than using which, could we switch this to dist/bin/scalac and dist/bin/scala and let the test fail if those aren't there or aren't executable?

I will investigate, sounds reasonable.

[dwijnand]

Thanks!!