[LL FIR] rename testdata to testData to avoid copyright generation

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/inBlockModification/extensionWithDefaultParametersScript.kts

@@ -0,0 +1,7 @@
+class MyProducer {
+    fun produce(): Int = 4
+}
+
+fun MyProducer.testFun(param1: Int = produce()) {
+    <expr>42</expr>
+}