test_runner: remove root tracking set

The wasRootSetup Set in the test harness appears to be
redundant, since the startTime field can be used interchangeably.
This commit removes wasRootSetup.

lib/internal/test_runner/harness.js

@@ -26,7 +26,6 @@ const {
 const { bigint: hrtime } = process.hrtime;
 
 const testResources = new SafeMap();
-const wasRootSetup = new SafeWeakSet();
 
 function createTestTree(options = kEmptyObject) {
   return setup(new Test({ __proto__: null, ...options, name: '<root>' }));
@@ -105,7 +104,7 @@ function collectCoverage(rootTest, coverage) {
 }
 
 function setup(root) {
-  if (wasRootSetup.has(root)) {
+  if (root.startTime !== null) {
     return root;
   }
 
@@ -164,8 +163,6 @@ function setup(root) {
   }
 
   root.startTime = hrtime();
-
-  wasRootSetup.add(root);
   return root;
 }