RFR: JDK-8245281 Disabling hotspot gtest builds make it impossible to run tests

[Magnus Ihse Bursie]

With JDK-8244757, the rule for test-image-hotspot-gtest is not defined 
if hotspot gtest builds are disabled using configure 
--disable-hotspot-gtest. This makes "make test-image" (and hence, "make 
test") fail.

The logic behind JVM_TEST_IMAGE_TARGETS was not really apparent to me, 
so I had to do some code archaeology to understand it's origins.  It 
came from JDK-8198425, which was an contribution from Steve Groeger.

I rewrote the code a bit, since I did not think it proper to try to tack 
on neither Graal nor Gtest test builds if this variable is used. The 
intention is basically to oveerride the set of JVM tests, not to create 
a new baseline to append to. Hopefully this intention comes across a bit 
clearer now.

Steve, I'd appreciate it if you can test this patch. Or, if you are not 
using the JVM_TEST_IMAGE_TARGETS extension point anymore, please let me 
know!

/Magnus

Bug: https://bugs.openjdk.java.net/browse/JDK-8245281
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8245281-hotspot-gtest-image/webrev.01