Extend test `trait-private-var` with vals

Private vars and private vals defined in a trait impact the bytecode
generated for the implementors of these traits. The scripted test
`source-dependencies/trait-private-var` only accounted for private vars.

It now tries the same scenario both with private vars and private vals.

sbt/src/sbt-test/source-dependencies/trait-private-var/changes/A2.scala

@@ -0,0 +1,5 @@
+trait A {
+	private val foo = 12
+	// we need to access foo to trigger AbstractMethodError
+	def bar: Int = foo
+}

sbt/src/sbt-test/source-dependencies/trait-private-var/test

@@ -1,14 +1,28 @@
+$ copy-file changes/A0.scala A.scala
+
 # compile and run for the first time to verify that everything works
 > run
 
 # introduce private var and refer to it in a trait that we inherit from
 # there'll be pair of getters and setters generated for private var that
 # has to be implemented by a class (where you can declare corresponding field)
-$ copy-file changes/A.scala A.scala
+$ copy-file changes/A1.scala A.scala
 
-# this fails with AbstractMethodError because getters and setters for
-# a private var are not generated because introduction of a private var
-# does not trigger recompilation of B
-# B is not recompiled because incremental compiler tracks only public
-# interace (members visible from outside of given trait/class)
+# If the introduction of a private var did not trigger the recompilation of B,
+# then this will fail with AbstractMethodError because the getters and setters
+# for the private var have not been generated.
+> run
+
+# Try again with a private val
+> clean
+
+$ copy-file changes/A0.scala A.scala
+
+# compile and run a clean project to verify that everything works
+> run
+
+# introduce a private val in the trait
+$ copy-file changes/A2.scala A.scala
+
+# Verify that B has been recompiled and that everything runs fine.
 > run