Disable MPScriptTestCase in the 'alt-dist' test jobs

The 'alt-dist' jobs like https://ci.wildfly.org/viewType.html?buildTypeId=WF_MasterLinuxAltDistJdk8 configure the TS to use the output of the ee-build/ee-dist modules or to provision using the wildfly-ee FP. MPScriptTestCase can't pass in such a scenario as the enable-microprofile.cli script it wants to run does not exist. So use a JUnit Assume to disable it. Checking for -Dtestsuite.default.build.project.prefix=ee- would probably do the trick.