Validate syscall_timing test names before starting to provide earlier

Validate syscall_timing test names before starting to provide earlier
feedback regarding user error.

Provide default loop and timing settings.

Add a new test that just times pread() without the open()/close().

Mark tests requiring a path argument so we can provide better feedback
to the user than EFAULT on (null).

Sponsored by:	Google, Inc.
MFC after:	2 weeks