Play IDE plugin: Migrate integration tests

[bmuschko]

• Migrate existing integration tests as much as possible
• Multi-version testing is not supported
• All tests should pass

[bmuschko]

Implementation:

• https://github.com/gradle/play/blob/master/src/integTest/groovy/com/lightbend/play/application/PlayIdePluginIntegrationTest.groovy
• https://github.com/gradle/play/blob/master/src/integTest/groovy/com/lightbend/play/application/PlayIdeaPluginIntegrationTest.groovy
• https://github.com/gradle/play/blob/master/src/integTest/groovy/com/lightbend/play/application/basic/PlayIdeaPluginBasicIntegrationTest.groovy
• https://github.com/gradle/play/blob/master/src/integTest/groovy/com/lightbend/play/application/advanced/PlayIdeaPluginAdvancedIntegrationTest.groovy
• https://github.com/gradle/play/blob/master/src/integTest/groovy/com/lightbend/play/application/multiproject/PlayIdeaPluginMultiprojectIntegrationTest.groovy

[bmuschko]

@big-guy I'd like to point you to two issues I encountered during the conversion:

• I ignored this test as the number of Scala SDK URLs doesn't match the expected number. I am not sure why that is happening.
• The expected source directories here don't seem to make sense to me.