Cygwin: align testsuite tmp directory with runtime path resolution

[aswin-mcw]

Cygwin: align testsuite tmp directory with runtime path resolution

Summary

While investigating Cygwin testsuite failures in MSYS2, I found that the installation root was being resolved one directory too far back in my build layout.

In init_cygheap::init_installation_root() (winsup/cygwin/mm/cygheap.cc), the __MSYS__ logic strips three directory levels in total when determining the installation root. For my test environment, this resulted in incorrect path resolution.

The testsuite was also creating temporary files under ${builddir}/testinst/tmp. Changing it to use ${builddir}/tmp aligns the test layout with the runtime's expected path resolution.

-$(MKDIR_P) ${builddir}/testinst/tmp
+$(MKDIR_P) ${builddir}/tmp

-rm -rf ${builddir}/testinst/tmp
+rm -rf ${builddir}/tmp

Results

After making this change, the winsup testsuite results improved significantly in my environment:

• Before: ~148 passing tests out of 282
• After: ~258 passing tests out of 282

This suggests that a large number of failures were caused by path resolution and test layout issues rather than individual runtime failures.