[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/compoundAssignWithArrayAccessConvention.kt

@@ -0,0 +1,8 @@
+interface MyMap<K, V> {
+    operator fun get(k: K): V
+    operator fun set(k: K, v: V)
+}
+
+fun test(m: MyMap<String, Int>) {
+    <expr>m["a"] += 1</expr>
+}