cmd/go: convert some coverage tests to the script framework

cmd/go: convert some coverage tests to the script framework

This change converts TestCoverageFunc, TestCoverageDashC,
TestCoverageSyncAtomicImport, TestCoverageErrorLine,
TestCoverageDepLoop, TestCoverageDotImport, and
TestTestBuildFailureOutput to the script framework.

It adds a -exec modifier to the script framework's [exists] check
to check that a file is executable.

Updates #17751

Change-Id: Idc1e36c3702c94918631936f637510a6679d18a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/212624
Reviewed-by: Jay Conrod <jayconrod@google.com>