check: add support for an external file containing tests to exclude

check: add support for an external file containing tests to exclude

Currently the -X option is intended to specify a set of expunging
files which are stored in each test/* subdirectory.  As described in
the commit description for 0b1e8abd4, in order to exclude the test
generic/280, the -X option is used as follows:

    $ cat tests/generic/3.0-stable-avoid
    280
    $ sudo ./check -X 3.0-stable-avoid generic/280

However, it is sometimes useful to store the set of expunged tests in
a single file, outside of tests/* subdirectories.  This commit enables
the following:

    $ cat /root/conf/data_journal.exclude
    generic/068
    ext4/301
    $ sudo ./check -E /root/conf/data_journal.exclude -g auto

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>