Fix "compile final vals in annotations" test in PluginASTSupprtTest.

This test validated if there is no error/warning related to [issue #54](#45) during compilation with Scoverage enabled.

Scala 2.13.0-M3 emits different warning "@serialversionuid has no effect on non-serializable classes".
This is unrelated to that issue and makes sense.

The simple fix is to use case class, which is serializable.

Author: gslowikowski

scalac-scoverage-plugin/src/test/scala/scoverage/PluginASTSupportTest.scala

@@ -165,7 +165,7 @@ class PluginASTSupportTest
                           |  final val foo = 1L
                           |}
                           |@SerialVersionUID(Foo.foo)
-                          |class Bar
+                          |case class Bar()
                           |""".stripMargin)
     assert(!compiler.reporter.hasErrors)
     assert(!compiler.reporter.hasWarnings)