Initialize inner gene for back reference gene during sampling for tests.

Author: lmasroca

core/src/test/kotlin/org/evomaster/core/search/gene/GeneSamplerForTests.kt

@@ -406,9 +406,12 @@ object GeneSamplerForTests {
     }
 
     fun sampleBackReferenceRxGene(rand: Randomness): BackReferenceRxGene {
+        val captureGroup = sampleDisjunctionListRxGene(rand)
+        // as we do not allow to mutate the inner captureGroup gene using the backref gene we must first initialize it
+        captureGroup.doInitialize(rand)
         return BackReferenceRxGene(
             groupIndex = rand.nextInt(1, 99),
-            captureGroup = sampleDisjunctionListRxGene(rand)
+            captureGroup = captureGroup
         )
     }