Clean up Beanshell and JavaScript unit tests

Firstly, the imagej.script.JavascriptTest in ij-core's test area was
merged into the ij-scripting-javascript component.

Secondly, the BeanshellTest was fixed to avoid referencing
imagej.script.DummyService in favor of its own DummyService (Eclipse
might allow such shenanigans, but from a perspective of proper Maven
dependencies the BeanshellTest was failing before this change).

Thirdly, the "testBasic" methods which actually tested the script's IJ
context were renamed to "testContext" to avoid ambiguity with the
testBasic methods that merely tested a simple addition operation.