selftests/powerpc/pmu: Add selftest for event alternatives for power9

Platform specific PMU supports alternative event for some of the event
codes. During perf_event_open, it any event group doesn't match
constraint check criteria, further lookup is done to find alternative
event. Code checks to see if it is possible to schedule event as group
using alternative events.

Testcase exercises the alternative event find code for power9. Example,
since events in same PMC can't go in as a group, ideally using
PM_RUN_CYC_ALT (0x200f4) and PM_BR_TAKEN_CMPL (0x200fa) will fail. But
since RUN_CYC (0x600f4) is alternative event for 0x200f4, it is possible
to use 0x600f4 and 0x200fa as group. Testcase uses such combination for
all events in power9 which has an alternative event.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610134113.62991-27-atrajeev@linux.vnet.ibm.com