Add test to confirm leak is gone

The following commit adds the test case used to report #3143 in a big
projects with lots of cross versions and modules. The execution of the
test is instant!

sbt/src/sbt-test/project/cross-source/build.sbt

@@ -0,0 +1,8 @@
+val commonSettings = Seq(
+  crossScalaVersions := (0 to 6).map(i => s"2.10.$i") ++ (0 to 11).map(i => s"2.11.$i") ++ (0 to 2).map(i => s"2.12.$i")
+)
+
+val p1 = project.in(file("p1")).settings(commonSettings)
+val p2 = project.in(file("p2")).settings(commonSettings)
+val p3 = project.in(file("p3")).settings(commonSettings)
+val p4 = project.in(file("p4")).settings(commonSettings)

sbt/src/sbt-test/project/cross-source/p1/src/main/scala-2.10/B.scala

@@ -0,0 +1,3 @@
+object B {
+  def show(what: String): String = s"String interpolation is ${what.toUpperCase}!"
+}

sbt/src/sbt-test/project/cross-source/p1/src/main/scala/A.scala

@@ -0,0 +1,3 @@
+class A {
+  def show(what: String): Unit = println(what)
+}

sbt/src/sbt-test/project/cross-source/p2/src/main/scala-2.10/B.scala

@@ -0,0 +1,3 @@
+object B {
+  def show(what: String): String = s"String interpolation is ${what.toUpperCase}!"
+}

sbt/src/sbt-test/project/cross-source/p2/src/main/scala/A.scala

@@ -0,0 +1,3 @@
+class A {
+  def show(what: String): Unit = println(what)
+}

sbt/src/sbt-test/project/cross-source/p3/src/main/scala-2.10/B.scala

@@ -0,0 +1,3 @@
+object B {
+  def show(what: String): String = s"String interpolation is ${what.toUpperCase}!"
+}

sbt/src/sbt-test/project/cross-source/p3/src/main/scala/A.scala

@@ -0,0 +1,3 @@
+class A {
+  def show(what: String): Unit = println(what)
+}

sbt/src/sbt-test/project/cross-source/p4/src/main/scala-2.10/B.scala

@@ -0,0 +1,3 @@
+object B {
+  def show(what: String): String = s"String interpolation is ${what.toUpperCase}!"
+}

sbt/src/sbt-test/project/cross-source/p4/src/main/scala/A.scala

@@ -0,0 +1,3 @@
+class A {
+  def show(what: String): Unit = println(what)
+}

sbt/src/sbt-test/project/cross-source/test

@@ -0,0 +1,3 @@
+# https://github.com/sbt/sbt/issues/3143
+> crossScalaVersions
+> +version