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

[Aleksey Shipilev]

On 03/22/2018 10:37 PM, Paul Sandoz wrote:
> Test looks good, but do you need to declare the source/target for compilation?

Thanks! source/target are from the times when default source/target was not 9, all other tests have
it (I copypasted the test from the existing ones). We should probably ditch those source/target-s
all at once soon.

Thanks,
-Aleksey