Add eval-mutable-variables test

Author: adpi2

tests/debug/eval-mutable-variables.check

@@ -0,0 +1,33 @@
+break A 12
+eval x = 2
+result ()
+eval x
+result 2
+eval
+  u = 2
+  u
+result 2
+eval u
+result 2
+eval y += 1
+result ()
+eval yy()
+result 3
+eval (new B).zz()
+result 2
+
+break A 14
+eval y+=1; y
+result 4
+
+break A 15
+eval y
+result 5
+
+break A$B$1 18
+eval z += 1; z
+result 3
+
+break A$B$1 19
+eval z
+result 4

tests/debug/eval-mutable-variables.scala

@@ -0,0 +1,21 @@
+object Test:
+  def main(args: Array[String]): Unit =
+    val a = new A
+    println(a.m)
+
+class A:
+  private var x = 1
+  def m: Int =
+    var y = 1
+    var z = 1
+    var u = 1 // not captured
+    x += 1
+    def yy(): Int =
+      y += 1
+      y
+    class B:
+      def zz(): Int =
+        z += 1
+        z
+    val b = new B
+    x + yy() + b.zz() + u