[OpenJDK 2D-Dev] RFR: 8183351: Better cleanup for jdk/test/javax/imageio/spi/AppletContextTest/BadPluginConfigurationTest.sh

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8183351
Webrev: http://cr.openjdk.java.net/~prr/8183351/

This bug caused an internal test system to report a failure of this test
when the userid used to run the tests changed due to lack of this cleanup,
so I'd like to get this one in soon.

It is a surprise that it has not caused such a problem in the past since
the test has existed for a long time - even though it was only made open
earlier this year.

NB mktemp is already used in other jtreg tests and this updated test
is passing on all platforms (windows, mac, linux, solaris).

-phil.