RootFS: Upgrade testsuite

[staticfloat]

This adds a few small improvements to our testsuite:

• It tests linking with both $CC and $FC when running the fortran test suite, to catch things like syslibroot for macOS needs to be manually fixed also for GCC v5 BinaryBuilder.jl#630

• It adds an OpenMP test to the c test suites, so that I can try out linking against different versions of libgomp.

[giordano]

This looks good to me!

There is one thing that was down in my TODO list that however can be done here since we're upgrading the testsuite: can we move test/build_tests/cxxstringabi_tests/lib.cc from BinaryBuilder to here so that C++ string ABI checking is actually exercised when running the Shard testsuite? Code coverage shows that C++ string ABI detection is completely untested.

[staticfloat]

That should already be exercised by the cxx/dyn_link test suite. If coverage is showing it as not being run, something else is going wrong.

[staticfloat]

The link you posted shows that detect_cxxstring_abi seems to be getting run (although we're never going down the "bad" paths; I think we need to run the test with a purposefully mismatched CompilerABI)

[giordano]

Ok, yes, we need better tests in BinaryBuilder, but the testsuite here should be fine