Add shell tests that run pmcstat(8) itself.

pmcstat_test.sh checks the brace-list syntax end to end: it runs
pmcstat with grouped events and checks the output columns, including
the residency column and the "<not counted>" marker. check-root runs
it when it is present.

pmcstat_mux_warning_test.sh checks the warning policy for a long
grouped sampling period. The warning depends on what the driver
reports, so the test preloads a shim that returns controlled values.
This lets it check that the warning appears only when multiplexing was
observed, not merely because the brace syntax set PMC_F_GROUP_MUX.

Sponsored by: AMD

Signed-off-by: Andre Silva <andasilv@amd.com>