antlr4-maven-plugin tests fail

How to reproduce:
```
mvn install -DskipTests
cd antlr4-maven-plugin
mvn test

```
Fails with:
[ERROR] Errors:
[ERROR] Antlr4MojoTest.importTokens:59 » NullPointer Cannot invoke "org.apache.maven.model.Plugin.getGroupId()" because the return value of "org.apache.maven.plugin.MojoExecution.getPlugin()" is null
[ERROR] Antlr4MojoTest.importsCustomLayout:120 » NullPointer Cannot invoke "org.apache.maven.model.Plugin.getGroupId()" because the return value of "org.apache.maven.plugin.MojoExecution.getPlugin()" is null
[ERROR] Antlr4MojoTest.importsStandardLayout:221 » NullPointer Cannot invoke "org.apache.maven.model.Plugin.getGroupId()" because the return value of "org.apache.maven.plugin.MojoExecution.getPlugin()" is null
[ERROR] Antlr4MojoTest.processWhenDependencyRemoved:326 » NullPointer Cannot invoke "org.apache.maven.model.Plugin.getGroupId()" because the return value of "org.apache.maven.plugin.MojoExecution.getPlugin()" is null
[INFO]
[ERROR] Tests run: 4, Failures: 0, Errors: 4, Skipped: 0
[
I noted that there is a mismatch between the pom and the src/test hierarchy.
It seems likely these tests were broken a long time ago, because our CI doesn't run tests for the maven plugin...