[FIR-TEST] Move analysis tests to separate module

compiler/fir/analysis-tests/testData/resolveWithStdlib/listPlusAssign.kt

@@ -0,0 +1,19 @@
+fun List<String>.modify() {
+    <!VARIABLE_EXPECTED!>this<!> += "Alpha"
+    <!VARIABLE_EXPECTED!>this<!> += "Omega"
+}
+
+fun Any.modify() {
+    (<!VARIABLE_EXPECTED!>this as List<Int><!>) += 42
+}
+
+operator fun <T> Set<T>.plusAssign(x: T) {}
+
+fun Set<String>.modify() {
+    this += "Alpha"
+    this += "Omega"
+}
+
+fun Any.modifySet() {
+    (this as Set<Int>) += 42
+}