[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/compoundAssignWithArrayGetConvention_lhs.kt

@@ -0,0 +1,11 @@
+interface A {
+    operator fun plusAssign(i: Int)
+}
+
+interface MyMap<K, V> {
+    operator fun get(k: K): V
+}
+
+fun test(m: MyMap<String, A>) {
+    <expr>m["a"]</expr> += 1
+}