EclipseJdtApt#setGenTestSrcDir sets the wrong underlying property

[lkoe]

https://github.com/diffplug/goomph/blob/main/src/main/java/com/diffplug/gradle/eclipse/apt/EclipseJdtApt.java#L117

The EclipseJdtApt#setGenTestSrcDir(Object) method targets the wrong extension property - it should be genTestSrcDir rather than genSrcDir.

I'll provide a PR shortly.