fix(blob): store startTime as a number (#24620)

Turns out the Date objects have noticeable footprint on large suites and
storing them as umber is much cheaper, e.g.:


![image](https://github.com/microsoft/playwright/assets/9798949/539028d0-3ef8-46f7-be2b-752f24604d18)

packages/playwright-test/src/isomorphic/teleReceiver.ts

@@ -87,7 +87,7 @@ export type JsonTestResultStart = {
   retry: number;
   workerIndex: number;
   parallelIndex: number;
-  startTime: string;
+  startTime: number;
 };
 
 export type JsonAttachment = Omit<TestResult['attachments'][0], 'body'> & { base64?: string };
@@ -105,7 +105,7 @@ export type JsonTestStepStart = {
   parentStepId?: string;
   title: string;
   category: string,
-  startTime: string;
+  startTime: number;
   location?: Location;
 };
 
@@ -232,7 +232,7 @@ export class TeleReporterReceiver {
     testResult.retry = payload.retry;
     testResult.workerIndex = payload.workerIndex;
     testResult.parallelIndex = payload.parallelIndex;
-    testResult.startTime = new Date(payload.startTime);
+    testResult.setStartTimeNumber(payload.startTime);
     testResult.statusEx = 'running';
     this._reporter.onTestBegin?.(test, testResult);
   }
@@ -510,22 +510,31 @@ class TeleTestStep implements TestStep {
   category: string;
   location: Location | undefined;
   parent: TestStep | undefined;
-  startTime: Date;
   duration: number = -1;
   steps: TestStep[] = [];
 
+  private _startTime: number = 0;
+
   constructor(payload: JsonTestStepStart, parentStep: TestStep | undefined, location:  Location | undefined) {
     this.title = payload.title;
     this.category = payload.category;
     this.location = location;
     this.parent = parentStep;
-    this.startTime = new Date(payload.startTime);
+    this._startTime = payload.startTime;
   }
 
   titlePath() {
     const parentPath = this.parent?.titlePath() || [];
     return [...parentPath, this.title];
   }
+
+  get startTime(): Date {
+    return new Date(this._startTime);
+  }
+
+  set startTime(value: Date) {
+    this._startTime = +value;
+  }
 }
 
 class TeleTestResult implements reporterTypes.TestResult {
@@ -533,7 +542,6 @@ class TeleTestResult implements reporterTypes.TestResult {
   parallelIndex: reporterTypes.TestResult['parallelIndex'] = -1;
   workerIndex: reporterTypes.TestResult['workerIndex'] = -1;
   duration: reporterTypes.TestResult['duration'] = -1;
-  startTime: reporterTypes.TestResult['startTime'] = new Date();
   stdout: reporterTypes.TestResult['stdout'] = [];
   stderr: reporterTypes.TestResult['stderr'] = [];
   attachments: reporterTypes.TestResult['attachments'] = [];
@@ -545,9 +553,23 @@ class TeleTestResult implements reporterTypes.TestResult {
   stepMap: Map<string, reporterTypes.TestStep> = new Map();
   statusEx: reporterTypes.TestResult['status'] | 'scheduled' | 'running' = 'scheduled';
 
+  private _startTime: number = 0;
+
   constructor(retry: number) {
     this.retry = retry;
   }
+
+  setStartTimeNumber(startTime: number) {
+    this._startTime = startTime;
+  }
+
+  get startTime(): Date {
+    return new Date(this._startTime);
+  }
+
+  set startTime(value: Date) {
+    this._startTime = +value;
+  }
 }
 
 export type TeleFullProject = FullProject & { id: string };

packages/playwright-test/src/reporters/teleEmitter.ts

@@ -199,7 +199,7 @@ export class TeleReporterEmitter implements ReporterV2 {
       retry: result.retry,
       workerIndex: result.workerIndex,
       parallelIndex: result.parallelIndex,
-      startTime: result.startTime.toISOString(),
+      startTime: +result.startTime,
     };
   }
 
@@ -229,7 +229,7 @@ export class TeleReporterEmitter implements ReporterV2 {
       parentStepId: (step.parent as any)?.[idSymbol],
       title: step.title,
       category: step.category,
-      startTime: step.startTime.toISOString(),
+      startTime: +step.startTime,
       location: this._relativeLocation(step.location),
     };
   }

tests/playwright-test/reporter.spec.ts

@@ -48,6 +48,7 @@ class Reporter {
   distillStep(step) {
     return {
       ...step,
+      _startTime: undefined,
       startTime: undefined,
       duration: undefined,
       parent: undefined,