[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/plainCheck.kt

@@ -0,0 +1,9 @@
+interface Foo {}
+
+fun test(obj: Any, another: Foo) {
+    if (obj is Foo) {
+        <expr>consume(another)</expr>
+    }
+}
+
+fun consume(obj: Foo) {}