oeqa/core/decorator/data.py: fix skipIfNotInDataVar

oeqa/core/decorator/data.py: fix skipIfNotInDataVar

The var might not be set, resulting in unexpected error.

  RESULTS - multilib.MultilibTest.test_check_multilib_libc - Testcase 1593: ERROR

The above error is due to MULTILIBS being not set, which is the default
for OE. This patch fixes this problem.

Also, the debugging message in skipIfNotInDataVar is currently confusing.
Instead of
DEBUG: Checking if 'MULTILIBS' value is in 'multilib:lib32' to run the test
it should be
DEBUG: Checking if 'MULTILIBS' value contains 'multilib:lib32' to run the test

This patch also fixes it.

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