Further hide methods in GradlePluginTestingStrategySpecs because IntelliJ wrongly suggest them

[lacasseio]

As IntelliJ is always annoying broken, it suggests methods in GradlePluginTestingStrategySpecs which is package private. I marked the methods inside a public as a means to know which methods are exported from the class at whatever visibility the class is but that doesn't seem to make much sense to IntelliJ.