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

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/contextCollector/contextReceiversClass.kt

@@ -0,0 +1,10 @@
+interface Foo {
+    fun foo(): Int
+}
+
+context(Foo)
+class Test {
+    fun test() {
+        <expr>foo()</expr>
+    }
+}