oeqa/runtime/cases/multilib.py: skip if needed packages are not available

1) The test cases use 'readelf' command to do the check. This command
   is from binutils. So skip the test if the needed binutils package is
   not installed.

   The related error message in log.do_testimage is like below.

     Output:  sh: readelf: not found

2) The test case tests /lib/libc.so.6 from lib32-libc6. So skip the test
   if lib32-libc6 is not installed.

   The related error message in log.do_testimage is like below.

     Output:   readelf: Error: 'lib/libc.so.6': No such file

(From OE-Core rev: eae929a5c224f5c3468d6a0466d1bbb3f678a5a1)

(From OE-Core rev: cb80105bbd03d8d9cc10139ed6f39b193e79c1b8)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/runtime/cases/multilib.py

@@ -27,6 +27,8 @@ class MultilibTest(OERuntimeTestCase):
     @skipIfNotInDataVar('MULTILIBS', 'multilib:lib32',
                         "This isn't a multilib:lib32 image")
     @OETestDepends(['ssh.SSHTest.test_ssh'])
+    @OEHasPackage(['binutils'])
+    @OEHasPackage(['lib32-libc6'])
     def test_check_multilib_libc(self):
         """
         Check that a multilib image has both 32-bit and 64-bit libc in.