As part of including tiered test definitions for JDK regression tests (JDK-8075543), it would be helpful if in jtreg there was some way to define an empty test group.

The particular use case is that we would like to define a full JDK tier N test run as

    jtreg ... jdk/:tierN langtools/:tierN jaxp/:tierN nashorn/:tierN

This should be valid, in other words not an error, even if a given repository does not defined any tier N tests.

For example, currently langtools contains no tier 2 tests and jaxp contains no tier 1 tests, but it is awkward to need to encode that kind of logic into test execution scripts.