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

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/getOrBuildFir/expressions/arrayIndexExpressionWithInc.kt

@@ -0,0 +1,6 @@
+// WITH_STDLIB
+fun main(args: Array<String>) {
+    val a = ArrayList<String>()
+    a.add("")
+    a[<expr>0</expr>]++
+}