Create Tests for views-markup feature

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
```

Start with build.gradle line 25 and the views-markup feature involved in creating compileGmlViews and MarkupViewCompilerTask. Reproduce the 7.0.0-M4 failure, then add tests that cover this task creation path and verify the reported fileExtension error is caught.

Written by the indexing model from the issue text.