perf test shell diff: Skip test if test_loop symbol is missing in the perf binary

The diff test depends on finding the symbol test_loop in perf and will
fail if perf has been stripped and no debug object is available. In that
case, skip the test instead.

Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231205164924.835682-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

perf test: Add basic 'perf diff' test

There are some old bug reports on perf diff crashing:

https://rhaas.blogspot.com/2012/06/perf-good-bad-ugly.html

Happening across them I was prompted to add two very basic tests that
will give some 'perf diff' coverage.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231120190408.281826-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>