Support more than one test file per license

[goneall]

Propose to test not just
test/simpleTestForGenerator/${LICENSE}.txt
but also
test/simpleTestForGenerator/${LICENSE}.*.txt
files. If they exists.

Reference spdx/license-list-XML#2635 proposed by @xsuchy

[goneall]

Note that we already have an implementation that supports multiple license tests - https://github.com/spdx/LicenseListPublisher/blob/master/src/org/spdx/licenselistpublisher/licensegenerator/LicenseTester.java

 * Test SPDX licenses against a directory of test licenses.
 *
 * The directory of test licenses contains license text with the following file naming convention:
 *
 * {license-id}/(license|header|exception)/(good|bad)/{test-id}.txt

[goneall]

Note that the LicenseXmlTester currently does not use the LicenseTester interface - this should be changed to use the common code as the publisher.