Assign all tests added via MY_ADD_TEST to a bogus default_ignore target,
so that they are not ran by default when doing bare make test. Add default
test named MTR that calls mysql-test-run suite, which is now the single
test run by make test.

In consequence, modified unit/suite.pm to exclude the MTR test and run the
real ctests flagged for default_ignore target, thus no circular
loop.