selftests/x86/ldt_gdt: Run most existing LDT test cases against the GDT as well

Now that the main test infrastructure supports the GDT, run tests
that will pass the kernel's GDT permission tests against the GDT.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/686a1eda63414da38fcecc2412db8dba1ae40581.1509794321.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>