Fix rewrite test: -source 3.0-migrate does not imply noindent

The code might have been rewritten with `-indent`, but still
needs to compile in scala2compat mode.

Author: liufengyun

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -197,7 +197,6 @@ object Scanners {
     val noindentSyntax =
       ctx.settings.noindent.value
       || ctx.settings.oldSyntax.value
-      || migrateTo3
     val indentSyntax =
       ((if (Config.defaultIndent) !noindentSyntax else ctx.settings.indent.value)
        || rewriteNoIndent)

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -71,7 +71,7 @@ class CompilationTests extends ParallelTesting {
     implicit val testGroup: TestGroup = TestGroup("rewrites")
 
     aggregateTests(
-      // compileFile("tests/rewrites/rewrites.scala", scala2CompatMode.and("-rewrite")),
+      compileFile("tests/rewrites/rewrites.scala", scala2CompatMode.and("-rewrite", "-indent")),
       compileFile("tests/rewrites/i8982.scala", defaultOptions.and("-indent").and("-rewrite"))
     ).checkRewrites()
   }

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -81,6 +81,13 @@ trait ParallelTesting extends RunnerOrchestration { self =>
       else self
     }
 
+    def withoutFlags(flags1: String*): TestSource = self match {
+      case self: JointCompilationSource =>
+        self.copy(flags = flags.without(flags1: _*))
+      case self: SeparateCompilationSource =>
+        self.copy(flags = flags.without(flags1: _*))
+    }
+
     /** Generate the instructions to redo the test from the command line */
     def buildInstructions(errors: Int, warnings: Int): String = {
       val sb = new StringBuilder