Provide utility to reset the Logback logging system

[sleberknight]

Provide a way to easily reset the Logback logging system, for example from an @AfterAll method:

@AfterAll
static void resetLogback() {
    LogbackTestHelpers.resetLogback();  // uses default logback-test.xml

    // could provide an overload too:
    LogbackTestHelpers.resetLogback("acme-special-logback-test.xml");
}

The final solution might not look like the above, i.e., the class name may be different, maybe we use instance methods instead of static, etc.

For more background see #457