Provide log4j2 replacement for StringMatchFilter

The current log4j1 StringMatchFilter provided through the hippo-repository-testutils module (REPO-572) requires a replacement for log4j2.
As a generic testutils utility, the new implementation will now be provided through the hippo-cms-testutils module (which the hippo-repository-testutils depends on).

Typical usage replacing the old StringMatchFilter in log4j.xml:

<filter class="org.onehippo.repository.testutils.log4j.StringMatchFilter"/>

to:

<StringMatchFilter/>