test suite: way to run single test file

test suite: way to run single test file

Added support for a new target in the test Makefile
to run a single test module.  If you do (for example)

make test-single TEST_FILES=lib/KohaTest/Biblio.pm

only the tests in that module will be run.  Unlike
the full test suite as run via 'make test', 'make test-single'
does not clear the test database before running the
tests.

Please note that "TEST_FILES=path/to/test/class.pm" is
required when using 'make test-single'.

Signed-off-by: Andrew Moore <andrew.moore@liblime.com>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>