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

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/contextCollector/smartCasts/insideLetOnNullableReceiver.kt

@@ -0,0 +1,7 @@
+fun foo(param: String?): String {
+    param?.let {
+        return <expr>param</expr>
+    }
+
+    return ""
+}