selftests/nolibc: support a test definition format

It now becomes possible to pass a string either in argv[1] or in the
NOLIBC_TEST environment variable (the former having precedence), to
specify which tests to run. The format is:

   testname[:range]*[,testname...]

Where a range is either a single value or the min and max numbers of the
test IDs in a sequence, delimited by a dash. Multiple ranges are possible.
This should provide enough flexibility to focus on certain failing parts
just by playing with the boot command line in a boot loader or in qemu
depending on what is accessible.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>