Extract coverage test helper functions into a separate shell script.

[c-mita]

Removes a lot of the duplication across different coverage shell tests, which all want similar functions.

[c-mita]

This also removes the check to disable tests if LLVM's gcov tool is used (since we use the same version check as the collect_cc_coverage.sh script).

In theory, these tests should be supported, although our test platform doesn't run them AFAIK.

[c-mita]

This also removes the check to disable tests if LLVM's gcov tool is used (since we use the same version check as the collect_cc_coverage.sh script).

In theory, these tests should be supported, although our test platform doesn't run them AFAIK.

I've restored the check against llvm-cov; there seems to be an issue with Mac test runs, and the LLVM paths in collect_cc_coverage are a little confusing. I think the wrong tool is being used in the wrong situation, and I need to investigate more.