htl: Enable but XFAIL tst-flock2, tst-signal1, tst-signal2

They need setpshared support.

* nptl/tst-flock2.c, tst-signal1.c, tst-signal2.c: Move to...
* sysdeps/pthread: ... here.
* nptl/Makefile: Move corresponding tests references to...
* sysdeps/pthread/Makefile: ... here.
* sysdeps/mach/hurd/i386/Makefile (test-xfail-tst-flock2,
test-xfail-tst-signal1, test-xfail-tst-signal2): Add.