test: Get the print mode from the CK_VERBOSITY environment variable

test: Get the print mode from the CK_VERBOSITY environment variable

On a test run it only prints the summary and one message per failed
test. While having this behaviour by default is nice it might be
interesting in some case to have more information print. Make the print
mode configurable from the environment variable CK_VERBOSITY, which can
have the values "silent", "minimal", "normal" or "verbose" so we can have
different outputs.

    $ sudo CK_VERBOSITY=verbose ./test-libevdev
    Running suite(s): libevdev init tests
     libevdev_has_event tests
     libevdev event tests
     100%: Checks: 79, Failures: 0, Errors: 0
     test-libevdev-init.c:23:P:test_new_device:test_new_device:0: Passed
     test-libevdev-init.c:28:P:test_free_device:test_free_device:0: Passed
     [ ... ]

Note that the default print mode doesn't change after this patch.

Signed-off-by: Enric Balletbo i Serra <eballetbo@redhat.com>