cross: Be more permissive about not-found exe_wrapper

We used to immediately try to use whatever exe_wrapper was defined in
the cross file, but some people generate the cross file once and use
it for several projects, most of which do not even need an exe wrapper
to build.

Now we're a bit more resilient. We quietly fall back to using
non-exe-wrapper paths for compiler checks and skip the sanity check.
However, if some code needs the exe wrapper, f.ex., if you run a built
executable using custom_target() or run_target(), we will error out
during setup.

Tests will, of course, continue to error out when you run them if the
exe wrapper was not found. We don't want people's tests to silently
"pass" (aka skip) because of a bad CI setup.

Closes https://github.com/mesonbuild/meson/issues/3562

This commit also adds a test for the behaviour of exe_wrapper in these
cases, and refactors the unit tests a bit for it.

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