iotests: add findtests.py

iotests: add findtests.py

Add python script with new logic of searching for tests:

Current ./check behavior:
 - tests are named [0-9][0-9][0-9]
 - tests must be registered in group file (even if test doesn't belong
   to any group, like 142)

Behavior of findtests.py:
 - group file is dropped
 - tests are all files in tests/ subdirectory (except for .out files),
   so it's not needed more to "register the test", just create it with
   appropriate name in tests/ subdirectory. Old names like
   [0-9][0-9][0-9] (in root iotests directory) are supported too, but
   not recommended for new tests
 - groups are parsed from '# group: ' line inside test files
 - optional file group.local may be used to define some additional
   groups for downstreams
 - 'disabled' group is used to temporary disable tests. So instead of
   commenting tests in old 'group' file you now can add them to
   disabled group with help of 'group.local' file
 - selecting test ranges like 5-15 are not supported more
   (to support restarting failed ./check command from the middle of the
    process, new argument is added: --start-from)

Benefits:
 - no rebase conflicts in group file on patch porting from branch to
   branch
 - no conflicts in upstream, when different series want to occupy same
   test number
 - meaningful names for test files
   For example, with digital number, when some person wants to add some
   test about block-stream, he most probably will just create a new
   test. But if there would be test-block-stream test already, he will
   at first look at it and may be just add a test-case into it.
   And anyway meaningful names are better.

This commit doesn't update check behavior (which will be done in
further commit), still, the documentation changed like new behavior is
already here.  Let's live with this small inconsistency for the
following few commits, until final change.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210125185056.129513-3-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>