Add Automatic-Module-Name for guava-testlib

[A248]

guava-testlib should have an Automatic-Module-Name for use in dependent modular projects. It seems that its module name, guava.testlib is derived from the filename as of 30.1.1.-jre. Issue https://github.com/google/guava/issues/2920 and related issues only added a module name for Guava itself.

While guava-testlib is most frequently used as a test-scoped dependency, there are cases where one may need to refer to its module name. Testing on the module path is possible. Sometimes this is necessary for IDE compatibility, as in https://github.com/ben-manes/caffeine/pull/540