Fix colltest5 test under separate compilation

This test failed before because strawman.collections.CollectionStrawMan5
is defined in two places:
- src/strawman/collections/CollectionStrawMan5.scala
- tests/run/colltest5/CollectionStrawMan5_1.scala

The first will be compiled by scalac (unless the tests are run through a
bootstrapped dotty) and the second will be compiled by dotty, the value
class encoding of scalac and dotty are not binary compatible.

This would not be a problem if we always used the `CollectionStrawMan5`
coming from the partest output directory and ignored the one in the
dotty sources, but which one gets picked depends on the classpath and whether compilation
is joined or separate, see #1301.

For now, it's safer and simpler to just avoid having tests which define
a class that is also defined in the sources of dotty.

Also, fix a bug in colltest4 where it was importing CollectionStrawMan5
instead of CollectionStrawMan4

Author: smarter

tests/run/colltest4/CollectionStrawMan4_1.scala

@@ -1,3 +1,4 @@
+package colltest4
 package strawman.collections
 
 import Predef.{augmentString => _, wrapString => _, _}

tests/run/colltest4/CollectionTests_2.scala

@@ -2,8 +2,8 @@ import Predef.{augmentString => _, wrapString => _, _}
 import scala.reflect.ClassTag
 
 object Test {
-  import strawman.collections._
-  import CollectionStrawMan5._
+  import colltest4.strawman.collections._
+  import CollectionStrawMan4._
 
   def seqOps(xs: Seq[Int]) = {
     val x1 = xs.foldLeft("")(_ + _)

tests/run/colltest5/CollectionStrawMan5_1.scala

@@ -1,3 +1,4 @@
+package colltest5
 package strawman.collections
 
 import Predef.{augmentString => _, wrapString => _, _}

tests/run/colltest5/CollectionTests_2.scala

@@ -2,7 +2,7 @@ import Predef.{augmentString => _, wrapString => _, _}
 import scala.reflect.ClassTag
 
 object Test {
-  import strawman.collections._
+  import colltest5.strawman.collections._
   import CollectionStrawMan5._
 
   def seqOps(xs: Seq[Int]) = {