Accordingly to jtreg's annotations in the test it should be run with ParallelGC only. Any attempts to run with other GC causes the following failure:

Conflicting collector combinations in option list; please refer to the release notes for the combinations allowed
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

The suggested fix is:

@@ -30,6 +30,7 @@
  * @summary Verifies that no partially initialized String object escapes from
  *          C2's String concat optimization in a highly concurrent setting.
  *          This test triggers the bug in about 1 out of 10 runs.
+ * @requires vm.gc == "null" | vm.gc == "Parallel"
  * @compile -XDstringConcat=inline TestStringObjectInitialization.java
  * @run main/othervm/timeout=300 -XX:+IgnoreUnrecognizedVMOptions -XX:-UseCompressedOops -XX:-CompactStrings
  *                               -XX:-UseG1GC -XX:+UseParallelGC TestStringObjectInitialization

It should prevent the test from being run under inappropriate GC.