kselftest/arm64: Try harder to generate different keys during PAC tests

We very intermittently see failures in the single_thread_different_keys
PAC test. As noted in the comment in the test the PAC field can be quite
narrow so there is a chance of collisions even with different keys with a
chance of 5% for 7 bit keys, and the potential for narrower keys. The test
tries to avoid this by running repeatedly, but only tries 10 times which
even with a 5% chance of collisions isn't enough.

Increase the number of times we attempt to look for collisions by a factor
of 100, this also affects other tests which are following a similar pattern
with running the test repeatedly and either don't care like with
pac_instruction_not_nop or potentially have the same issue like
exec_sign_all.

The PAC tests are very fast, running in a second or two even in emulation,
so the 100x increased cost is mildly irritating but not a huge issue. The
bulk of the overhead is in the exec_sign_all test which does a fork() and
exec() per iteration.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241111-arm64-pac-test-collisions-v1-2-171875f37e44@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

tools/testing/selftests/arm64/pauth/pac.c

@@ -13,7 +13,7 @@
 #include "../../kselftest_harness.h"
 #include "helper.h"
 
-#define PAC_COLLISION_ATTEMPTS 10
+#define PAC_COLLISION_ATTEMPTS 1000
 /*
  * The kernel sets TBID by default. So bits 55 and above should remain
  * untouched no matter what.