GroovyAssert should not depend on org.junit.Assert

This will make GroovyAssert a good companion to also JUnit5 without JUnit4 jar on the classpath.

Involves:

• References to assertTrue and fail should be removed (trivial)
• Hard-coded Test annotation check should be replaced with name check
• GroovyAssert should not extend org.junit.Assert. The methods in org.junit.Assert are static and can be statically imported directly.

For Groovy 4 only.