test framework - two improvements

test framework - two improvements

[1] When running the database-dependent tests (cd t, make test),
    all tables in the test database are dropped prior to running
    the installer and test cases.  This means that the test
    database will start with a clean slate.
[2] It is now possible to specify a single test class to run,
    to avoid having to run all of them:

    cd t
    make test TEST_CLASS=Search

    To run all DB-dependent tests, just do the usual

    cd t
    make test

Signed-off-by: Joshua Ferraro <jmf@liblime.com>