SI-5938 Test for a FSC bug with mixin, duplicate static forwarders

Under resident compilation, we were getting multiple copies of static
forwarders created for mixed in methods. It seems like the bug was
fixed as a by-product of scala#4040.

This commit adds a test to show this. I've confirmed that the test
fails appropriately with 2.11.4.

For future reference, before I figured out how to write the test
for this one (test/resident doesn't seem to let you run the code
after compiling it), I was using bash to test as follows:

    (export V=2.11.x; (scalac-hash $V sandbox/t5938_1.scala; (for i in 1 2; do echo sandbox/t5938.scala; done; printf '\n') | scalac-hash $V  -Xresident); stty echo; scala-hash $V X ; echo ':javap -public X' | scala-hash $V);

Author: retronym

test/files/run/t5938.scala

@@ -0,0 +1,35 @@
+import scala.tools.partest.DirectTest
+
+object Test extends DirectTest {
+
+  override def extraSettings: String =
+    s"-usejavacp -d ${testOutput.path}"
+
+  override def code = """
+object O extends C {
+  def main(args: Array[String]): Unit = {
+  }
+  // Static forwarder for foo and setter_foo_= added more once in a multi-run compile.
+}
+  """.trim
+
+  override def show(): Unit = {
+    val global = newCompiler()
+    Console.withErr(System.out) {
+      compileString(global)(code)
+      compileString(global)(code)
+      loadClass // was "duplicate name and signature in class X"
+    }
+  }
+
+  def loadClass: Class[_] = {
+    val cl = new java.net.URLClassLoader(Array(testOutput.toFile.toURL));
+    cl.loadClass("O")
+  }
+}
+
+trait T {
+  val foo: String = ""
+}
+class C extends T
+