RFR: JDK-8171163 Merge 9de6a70d5d81 broke test/Makefile

[Magnus Ihse Bursie]

The merge 9de6a70d5d81 (in jdk9/hs) reintroduced code that was deleted 
in JDK-8170629.

The result of this is that the target jtreg_tests is declared two times, 
which causes make to protest.

Note that the broken merge is in jdk9/hs, so this is where the fix needs 
to go.

Bug: https://bugs.openjdk.java.net/browse/JDK-8171163
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8171163-borked-merge-on-test-Makefile/webrev.01

/Magnus