[MCOMPILER-558] compileSourceRoots in testCompile should be writable (#209)

Make compileSourceRoots read/write so that it can be configured

Co-authored-by: Slawomir Jaranowski <[email protected]>

Author: lorenzsimon

Diff for: src/main/java/org/apache/maven/plugin/compiler/TestCompilerMojo.java

@@ -70,7 +70,7 @@ public class TestCompilerMojo extends AbstractCompilerMojo {
     /**
      * The source directories containing the test-source to be compiled.
      */
-    @Parameter(defaultValue = "${project.testCompileSourceRoots}", readonly = true, required = true)
+    @Parameter(defaultValue = "${project.testCompileSourceRoots}", readonly = false, required = true)
     private List<String> compileSourceRoots;
 
     /**