Such approach allows to run *.t scripts from any
tests/ subdirectories.

One point is that tests from tests/cls/*.t (which are needed
by tests/cls-testbed.t but does not exist yet) will also
be ran aside with tests/cls-testbed.t which is not good
because in such case they will be ran twice, so renamed these
tests path to tests/cls/*.c in tests/cls-testbed.t
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>