There are two problems here. The first is that we're doing manual monkey
patching which is fragile and verbose, when unittest.mock is designed
specifically to solve this exact problem. The second is that we're
monkey patching os.environ at two different levels with the same
information. So let's only do it once.

Add '--cross-only' option to run_tests.py, so we can arrange not to run
tests in the 'native' suite when only a cross-compiler is available, as
they can't succeed.

This bug not beeing caught in review is another reason for
adding strict typing support to meson.

on some systems, tests may take over an hour to run--only to find
you might have used an unintended Meson version (e.g. release instead
of dev). This change prints the Meson version at the start of the
run_*tests*.py scripts.

Also, raise SystemExit(main()) is preferred in general over
sys.exit(main())

Currently, colourize_console is a constant, set at process
initialization.

To allow the actual stdout to be easily compared with the expected when
running tests, we want to allow colourization to be on for the test
driver, but not for the in-process configure done by run_configure,
which has stdout redirected from a tty to a pipe.

v2:
Cache _colorize_console per file object

v3:
Reset cache on setup_console()

A current rather untyped storage of options is one of the things that
contributes to the options code being so complex. This takes a small
step in synching down by storing the compiler options in dicts per
language.

Future work might be replacing the langauge strings with an enum, and
defaultdict with a custom struct, just like `PerMachine` and
`MachineChoice`.

This is more correct, and forces the target(s) to be rebuilt if the
PDB files are missing. Increases the minimum required Ninja to 1.7,
which is available in Ubuntu 16.04 under backports.

We can't do the same for import libraries, because it is impossible
for us to know at configure time whether or not an import library will
be generated for a given DLL.

Pull the crossfile specification out of run_test.py so it can be
specified in the CI job configuration.

Also make some fixes to output ordering in run_test.py.