rt-tests: rt_bmark.py: fix TypeError

Fix following error:
  File "/usr/lib64/rt-tests/ptest/./rt_bmark.py", line 287, in run_cyclictest_once
    m = rex.search(line)
        ^^^^^^^^^^^^^^^^
TypeError: cannot use a string pattern on a bytes-like object

(From OE-Core rev: 9563027c35a4b1961a83100e22d4ea4430abd8b9)

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c5108da4009ccd3dfc92632171d6bc4dae4507db)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-rt/rt-tests/files/rt_bmark.py

@@ -284,7 +284,7 @@ def run_cyclictest_once():
         avg_cnt = 0
 
         for line in res.splitlines():
-                m = rex.search(line)
+                m = rex.search(line.decode('utf-8'))
                 if m is not None:
                         minlist.append(int(m.group(2)))
                         maxlist.append(int(m.group(4)))