[8u20] post-commit review: RT-36265: Disable robot tests by default for developers

[Kevin Rushforth]

I just pushed the fix for the following bug:

JIRA:  https://javafx-jira.kenai.com/browse/RT-36265
Changeset:  http://hg.openjdk.java.net/openjfx/8u-dev/rt/rev/15fdfaeb91f6

With this changeset, the default value of the gradle "USE_ROBOT" flag is 
now false. Use "-PUSE_ROBOT=true" to enable the robot-based tests. Note 
that this flag is only relevant when "-PFULL_TEST=true" (that hasn't 
changed).

-- Kevin