perf test: Check output of the probe ... --funcs command

Test "perf probe of function from different CU" only checks if the perf
command has failed and doesn't test the --funcs output. In the issue
reported in the previous commit, the garbage output of the --funcs
command was being ignored by the test when it could have been caught.

The script first makes use of --funcs option with the perf probe command
to check if the function "foo" exists in the testfile before adding a
probe to it in the next command. The output of probe...--funcs command
is redirected to stdout, therefore, add '| grep "foo"' to validate the
result.

Signed-off-by: Chaitanya S Prakash <chaitanyas.prakash@arm.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: anshuman.khandual@arm.com
Cc: james.clark@arm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240601125946.1741414-11-ChaitanyaS.Prakash@arm.com