Assume experimental compiler support in CommunityBuildTests

I noted that several community build tests (not just scodec) require experimental features.
We need to decide what the policy should be

 1. allow them by default, and tests can opt into non-experimental with -Yno-experimental?
 2. keep the standard policy, and don't run certain tests when run with an unstable compiler

If it's (2), we need a lot some more work on the community build to adapt all the tests that would
fail otherwise.

Author: odersky

community-build/test/scala/dotty/communitybuild/CommunityBuildTest.scala

@@ -6,10 +6,13 @@ import java.nio.charset.StandardCharsets.UTF_8
 import org.junit.{Ignore, Test}
 import org.junit.Assert.{assertEquals, fail}
 import org.junit.experimental.categories.Category
+import dotty.tools.dotc.config.Config
 
 abstract class CommunityBuildTest:
   given CommunityBuildTest = this
 
+  Config.compilerSupportsExperimental = true // individual tests can turn experimental off by setting -Yno-experimental
+
   /** Depending on the mode of operation, either
    *  runs the test or updates the project. Updating
    *  means that all the dependencies are fetched but
@@ -116,7 +119,7 @@ class CommunityBuildTestB extends CommunityBuildTest:
   @Test def disciplineSpecs2 = projects.disciplineSpecs2.run()
   @Test def munit = projects.munit.run()
   @Test def perspective = projects.perspective.run()
-  @Test def scodec = if (compilerSupportExperimental) projects.scodec.run()
+  @Test def scodec = if compilerSupportExperimental then projects.scodec.run()
   @Test def scodecBits = projects.scodecBits.run()
   @Test def simulacrumScalafixAnnotations = projects.simulacrumScalafixAnnotations.run()
 end CommunityBuildTestB