Add test for using switch version in multi command

I noticed that in 1.3.0-RC2, the following commands worked:
  ++2.11.12 ;test
  ;++2.11.12;test
but
  ++2.11.12; test
did not. This issue is fixed in the next commit.

sbt/src/sbt-test/actions/cross-multi-parser/build.sbt

@@ -0,0 +1 @@
+crossScalaVersions := Seq[String]("2.11.12", "2.12.8")

sbt/src/sbt-test/actions/cross-multi-parser/src/main/scala/cross/Build.scala

@@ -0,0 +1,3 @@
+package cross
+
+object Build

sbt/src/sbt-test/actions/cross-multi-parser/test

@@ -0,0 +1,5 @@
+> ++2.11.12; compile
+
+> ++ 2.12.8  ; compile;
+
+> ++ 2.12.8  ; compile