include warning in "No files created in test directory" test system output if no extensions were registered

The test system finds out about which files to compile by merging all EXTENSIONS options from all thorns. If none are registered it just reports "no files were generated" since no files match the patterns. It might be useful to include a hint "Did you add file extensions in any test.ccl file?" in the test system output. This issue can arise if for example only some tests are transferred to a cluster or one inserts test cases into a source tree from a formaline tarball (to verify if that particular code would still pass the test). The tests then appear to all pass since 0 out of 0 files compare identical.

Keyword: test
Keyword: system