Create Tests for views-markup feature

[jamesfredley]

No tests currently exist for this feature.

With 7.0.0-M4 the following error occurs and was missed since test coverage does not exist

FAILURE: Build failed with an exception.

* Where:
Build file 'build.gradle' line: 25

* What went wrong:
A problem occurred evaluating root project 'demo'.
> Failed to apply plugin 'org.apache.grails.gradle.grails-markup'.
   > Could not create task ':compileGmlViews'.
      > Could not create task of type 'MarkupViewCompilerTask'.
         > Cannot invoke "org.gradle.api.provider.Property.convention(Object)" because "this.fileExtension" is null

[jamesfredley]

Related to: #14804