ftrace/selftests: Update the direct call selftests to test two direct calls

The register_ftrace_direct() takes a different path if there's already a
direct call registered, but this was not tested in the self tests. Now that
there's a second direct caller test module, we can use this to test not only
one direct caller, but two.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>