tests: Improve check for /proc/cmdline kargs

tests: Improve check for /proc/cmdline kargs

On some systems there may be no root= argument, so the tests for
appending /proc/cmdline arguments will fail. Instead, loop over each of
the arguments in the host's /proc/cmdline and test that they're in the
constructed config file. That will actually test if ostree added all of
the system's /proc/cmdline args correctly. The regex isn't perfect here,
but it's probably good enough for this test.

Closes: #372
Approved by: cgwalters