While I'm not 100% sure of this, I think that some stuff that was committed around this issue has broken the GrailsConfiguration.
It seems that STS is no longer picking up the Grails install sitting alongside STS automatically anymore.
This is causing all of the Grails UI tests to fail because a Grails install cannot be found to execute Grails commands. It also means that I can no longer run any Grails related tests locally either.
For an example of the tests failing: see this screenshot from the SWTBot tests.
Notice the "No grails configuration ..." message displayed in the dialog.
Before the changes committed on that particular build, the Grails install was configured automatically if it was found in the right place. Now, it seems we have to manually configure Grails installs.