Validate test group names

In JDK-8238932 it was noted that one of the test groups was incorrectly defined:

tier1_gc_1 = \
  :gc_epsilon \
  gc/g1/ \
  -gc/g1/ihop/TestIHOPErgo.java
  -gc/g1/TestTimelyCompaction.java

Note specifically that there's a missing backslash on the second to last line. This means the last line will effectively define a test group with a rather interesting name ("-gc/g1/TestTimelyCompaction.java").

It would be good to investigate what can be done to improve jtreg to catch this type of error. For example, perhaps it's possible to validate the test group names to check that they don't contain unexpected characters ('-', '/', etc.)?