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

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/getOrBuildFir/kt60638.kt

@@ -0,0 +1,9 @@
+package usage
+
+class MyType
+class MyClass
+val MyClass.isInterface get() = 4
+
+fun usage(type: MyType) {
+    type.<expr>isInterface</expr>()
+}