Fix error after rebase

Author: abeln

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

@@ -289,8 +289,10 @@ class CompilationTests extends ParallelTesting {
   // Explicit nulls tests
   @Test def explicitNullsNeg: Unit = {
     implicit val testGroup: TestGroup = TestGroup("explicitNullsNeg")
-    compileFilesInDir("tests/explicit-nulls/neg", explicitNullsOptions) +
+    aggregateTests(
+      compileFilesInDir("tests/explicit-nulls/neg", explicitNullsOptions),
       compileFilesInDir("tests/explicit-nulls/neg-patmat", explicitNullsOptions and "-Xfatal-warnings")
+    )
   }.checkExpectedErrors()
 
   @Test def explicitNullsPos: Unit = {