Changed default to single-threaded test evaluator (com-lihaoyi#1074)

We see sporadic failures in CI, which I hope we can avoid by this.
These come in most cases from coursier or scalanative.
And current coursier is known to have some parallelization issues.

Pull request: com-lihaoyi#1074

main/test/src/util/TestEvaluator.scala

@@ -25,7 +25,7 @@ object TestEvaluator{
 class TestEvaluator(
   module: TestUtil.BaseModule,
   failFast: Boolean = false,
-  threads: Option[Int] = None
+  threads: Option[Int] = Some(1)
 )(implicit fullName: sourcecode.FullName,
   tp: TestPath
 ){