user_events: Add self-test for validator boundaries

Tests to ensure validator boundary cases are working correctly within
close and far bounds. Ensures __data_loc and __rel_loc strings are
null terminated and within range. Ensures min size checks work as
expected.

Link: https://lkml.kernel.org/r/20220118204326.2169-11-beaub@linux.microsoft.com



Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>