Use the TOOLEXT variable in run-tests.sh in the same way as in the
Makefile. The Makefile sets this variable automatically if invoked
by mingw32-make, but if running run-tests.sh in WSL with native
Windows compilers, then we need to set TOOLEXT manually when calling
run-tests.sh.
Allow skipping the cfguard failure tests when run in such a
configuration, since Linux make in WSL can't easily run cmd.exe
like in MSYS2.