Add default test suite argument

[andre2007]

This pull request allows to set the test suite name (currently fixed to dunit) via argument --defaultTestSuite.

In a later step, maybe it make sense to add a struct TestSuite which could be used as UDA on a class

struct TestSuite
{
string name;
}

Example usage:

@TestSuite("TestSuiteA")
class Test
{
    mixin UnitTest;

    @Test
    public void assertEqualsFailure() @safe pure
    {
        string expected = "bar";
        string actual = "baz";

        assertEquals(expected, actual);
    }