Incorrect scripted test for compiler plugin

[liufengyun]

As discovered in sbt/sbt#4641, the scripted test for compiler plugin has a problem:

• The plugin specifies that it runs before Staging and after Pickler

The requirement cannot be satisfied, thus an error is reported by the compiler -- which is the case in sbt/sbt#4641.

However, the scripted test failed to report the conflict -- it mistook the plugin-ordering error as an expected compilation error reported by the plugin.