Introduce the testsuite `--with-langkit-python` option so that it is
possible to have:
* one interpreter with the e3 distribution (to run the testsuite),
* one interpreter with Langkit installed (to run lkm and the various
scripts using Langkit modules),
* one bare interpreter to import Langkit-generated libraries.
When running all tests, restrict each test to a single core, otherwise
we end up running spamming the host with N*N subprocesses despite
--jobs=N passed to the testsuite. However, keep inner parallelism to N
when running few tests, for dev convenience.
In particular, rename "testsuite_support" to "drivers". This will make
it much more comfortable for developpers to use tab-completion in
shells, as there will no longer be a conflict between
"testsuite_support" and "tests" directories.
TN: T511-017
