Since using the ssp routines from mingw-w64 instead of those from
GCC's libssp, these messages are included in the piped output of
the process, so we can check for the right messages.
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.
Martin Storsjö