javax/imageio/plugins/wbmp/WBMPStreamTruncateTest.java fails if the test source tree is read-only as it tries to create a temporary file there instead of in the provided jtreg scratch dir.

Stacktrace:
java.io.IOException: Permission denied
at java.base/java.io.UnixFileSystem.createFileExclusively0(Native Method)
at java.base/java.io.UnixFileSystem.createFileExclusively(UnixFileSystem.java:218)
at java.base/java.io.File.createTempFile(File.java:2218)
at WBMPStreamTruncateTest.main(WBMPStreamTruncateTest.java:60)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
at java.base/java.lang.reflect.Method.invoke(Method.java:573)
at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
at java.base/java.lang.Thread.run(Thread.java:1576)