RFR: 8160505, Automated test runs fail in nashorn because TEST_IMAGE_DIR is set by jib

[Tim Bell]

All-

When using jib to run jtreg tests in an automated setting, 
TEST_IMAGE_DIR is set for those test suites having native code. Refer to 
jdk/test/Makefile or hotspot/test/Makefile for two examples.

The nashorn repo does not have any native tests, so 
nashorn/test/Makefile should not consider TEST_IMAGE_DIR.

Bug report:
    https://bugs.openjdk.java.net/browse/JDK-8160505

Webrev:
    http://cr.openjdk.java.net/~tbell/8160505/nashorn/webrev/

Tested by hand before and after applying the suggested fix.

Thanks in advance-

Tim