test: Prepare the test scripts for testing native Windows tools via WSL

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.

test/Makefile

@@ -252,8 +252,12 @@ ifeq ($(CMD),)
     FAILURE_TEST_TARGETS = $(addprefix .failtested., $(FAILURE_TESTS))
     CUSTOM1_FAILURE_TEST_TARGETS = .customtest1.bufferoverflow-fortify$(EXEEXT)
     ifneq ($(HAVE_CFGUARD),)
+    ifeq ($(WSL),)
+        # These tests do special calling of cmd.exe, which don't quite
+        # work when running with Linux make in WSL.
         CUSTOM2_FAILURE_TEST_TARGETS = .customtest2.cfguard-test$(EXEEXT)
     endif
+    endif
 endif
 endif