Allow condition in GILStatNode

Prevent the time stamps in the test runner from being captured together with the output of the C compiler.

Disable the new compiler warning about ambiguous exception return values when the function is external (and therefore possibly not under the control of the user).

Faster absolute reimports

When running a single test for multiple language levels, make sure they end up in different directories to prevent overwriting each other.

GH-2854: Implement CPython's safety guard against concurrent module initialisations during the import.

GH-2854: Reimplement absolute module imports to speed them up, especially under Py3.7. Cython used to be 3x slower than CPython here.
Also enable absolute imports automatically in top-level (non-package) modules.

Allow testing a single test file with both Cy2 and Cy3 language levels by giving it the new tag "all_language_levels".

This reverts commit d49062fd.

When detecting an invalid system configuration for overflow checks, at least print a warning, instead of leaving a dangling live exception behind.