Add tests for assignment operator

Author: cb372

tests/neg/warn-value-discard.check

@@ -14,3 +14,7 @@
 39 |    mutable.Set.empty[String].subtractOne("") // error
    |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |    discarded non-Unit value of type scala.collection.mutable.Set[String]
+-- [E172] Potential Issue Error: tests/neg/warn-value-discard.scala:59:4 -----------------------------------------------
+59 |    mutable.Set.empty[String] += "" // error
+   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |    discarded non-Unit value of type scala.collection.mutable.Set[String]

tests/neg/warn-value-discard.scala

@@ -51,3 +51,16 @@ class ValueDiscardTest:
     // - receiver is a local variable
     // --> No warning
     field.subtractOne("")
+
+  def assignmentOperator(): Unit =
+    // - += returns this.type
+    // - receiver is not a field or a local variable
+    // --> Warning
+    mutable.Set.empty[String] += "" // error
+
+  def assignmentOperatorLocalVariable(): Unit =
+    // - += returns this.type
+    // - receiver is a local variable
+    // --> No warning
+    val s: mutable.Set[String] = mutable.Set.empty[String]
+    s += ""