executing single test case from test suite

Hey,

so I just pushed a patch that allows you to execute just a single test case
from a whole suite.

It is mainly meant for the few cases where executing the whole suite takes
a long time (e.g. export tests with export validation enabled). Please use
it carefully and only during development. Check with a make check or
similar before pushing.

You can use that new feature through the environment variable
CPPUNIT_TEST_NAME which should contain the fully qualified name of the test
case that you want to run.

Examples mentioned in the commit are:

CPPUNIT_TEST_NAME="ScFiltersTest::testOutlineODS
ScFiltersTest::testRangeNameXLS" make
CppunitTest_sc_subsequent_filters_test

that one executes exactly 2 of the 75 tests in subsequent_filters_test.

CPPUNIT_TEST_NAME="ScFiltersTest" make CppunitTest_sc_subsequent_filters_test

would execute all ScFiltersTest cases which can be useful for the few
locations where we have several suites in a single library.

Feel free to comment and improve the implementation.

Regards,
Markus