[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/insidePlusAssignTarget.kt

@@ -0,0 +1,6 @@
+// WITH_STDLIB
+// LOOK_UP_FOR_ELEMENT_OF_TYPE: org.jetbrains.kotlin.psi.KtCallExpression
+
+fun foo(x: MutableMap<Int, MutableList<String>>) {
+    x.getOrPut(1) { <expr>mutableListOf<String>()</expr> } += "str"
+}