selftests/ftrace: Fix dependencies for some of the synthetic event tests

Commit b81a3a100cca1b ("tracing/histogram: Add simple tests for
stacktrace usage of synthetic events") changed the output text in
tracefs README, but missed updating some of the dependencies specified
in selftests. This causes some of the tests to exit as unsupported.

Fix this by changing the grep pattern. Since we want these tests to work
on older kernels, match only against the common last part of the
pattern.

Link: https://lore.kernel.org/linux-trace-kernel/20230614091046.2178539-1-naveen@kernel.org

Cc: <linux-kselftest@vger.kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Fixes: b81a3a100cca ("tracing/histogram: Add simple tests for stacktrace usage of synthetic events")
Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

selftests/ftrace: Add check for ping command for trigger tests

All these tests depend on the ping command and will fail if it is not
found. Allow tests to specify dependencies on programs through the
'requires' field. Add dependency on 'ping' for some of the trigger
tests.

Link: https://lore.kernel.org/all/20221017104312.16af5467@gandalf.local.home/
Reported-by: Akanksha J N <akanksha@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests/ftrace: Add test case for synthetic event dynamic strings

Add a selftest that defines and traces a synthetic event that uses a
dynamic string event field.

Link: https://lkml.kernel.org/r/74445afb005046d76d59fb06696a2ceaa164dec9.1601848695.git.zanussi@kernel.org

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>