Harmony: Refactor tests

Remove string signatures in favor of the already existing helper
functions.

Bug: 116674705
Test: mmma external/apache-harmony/jdwp
Test: m apache-harmony-jdwp-tests-hostdex vogar && art/tools/run-jdwp-tests.sh --mode=host
Change-Id: I0b54b83ebb1793a93418aa682dd136c88864cdd5