RFR 8200118: [test] String concat tests should test toString() application order

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8200118

Webrev:
  http://cr.openjdk.java.net/~shade/8200118/webrev.01/

This adds the relevant test to StringConcat family of tests, which asserts all strategies do the
right thing. There was a bug in MH_* strategies caused by the already fixed bug in filterArguments
[1], which this test was supposed to find, if existed before.

Testing: run-test with jdk/jdk, run-test before/after filterArguments fix in jdk-updates/jdk10u

Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8194554