[test] Skip a test when using an out-of-tree debugserver

The test "test_fp_special_purpose_register_read" in TestRegisters.py
fails on Darwin machines configured to use an out-of-tree debugserver.

The error message is: 'register read ftag' returns expected result, got
'ftag = 0x80'. This indicates that the debugserver in use is too old.

This commit introduces a decorator which can be used to skip tests which
rely on having a just-built debugserver. This resolves the issue:

$ ./bin/llvm-dotest -p TestRegisters.py -v
  1 out of 617 test suites processed - TestRegisters.py
  Test Methods:          7
  Success:               6
  Skip:                  1
...

llvm-svn: 327052