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

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/getOrBuildFir/calls/compoundAssignOnVal.kt

@@ -0,0 +1,6 @@
+interface A {
+    operator fun plusAssign(i: Int)
+}
+fun test(l: A) {
+    <expr>l += 1</expr>
+}