Fix build on NetBSD: Stop mixing PAL and system headers in tests

Fix build on NetBSD: Stop mixing PAL and system headers in tests

Including both system and PAL headers results in clasesh like:

In file included from /tmp/pkgsrc-tmp/wip/coreclr-git/work/coreclr/src/pal/tests/palsuite/threading/GetThreadTimes/test1/test1.c:18:
In file included from /usr/include/sched.h:37:
In file included from /usr/include/sys/sched.h:73:
/usr/include/sys/types.h:54:18: error: typedef redefinition with different types ('__int8_t' (aka 'signed char') vs 'char')
typedef __int8_t        int8_t;
                        ^
/tmp/pkgsrc-tmp/wip/coreclr-git/work/coreclr/src/pal/inc/pal_mstypes.h:224:16: note: previous definition is here
typedef __int8 int8_t;
               ^

This fixes #2993 "Conflicts with system headers on NetBSD"

Thanks to Jan Kotas (Microsoft) for showing the right direction.