ctest: Record StartTestTime entry for each test in Test.xml

Capture the time at which each test was started.  The time should be in
milliseconds to allow quick test runs to be differentiated.

Fixes: #26859

Source/CTest/cmCTestRunTest.cxx

@@ -365,6 +365,9 @@ cmCTestRunTest::EndTestResult cmCTestRunTest::EndTest(size_t completed,
     if (!skipped) {
       this->TestResult.CompletionStatus = "Completed";
     }
+    this->TestResult.StartTestTime =
+      std::chrono::duration_cast<std::chrono::milliseconds>(
+        this->TestProcess->GetSystemStartTime().time_since_epoch());
     this->TestResult.ExecutionTime = this->TestProcess->GetTotalTime();
     this->MemCheckPostProcess();
     this->ComputeWeightedCost();

Source/CTest/cmCTestTestHandler.cxx

@@ -1525,6 +1525,7 @@ void cmCTestTestHandler::WriteTestResultHeader(cmXMLWriter& xml,
   xml.Element("Path", this->CTest->GetShortPathToFile(result.Path));
   xml.Element("FullName", this->CTest->GetShortPathToFile(testPath));
   xml.Element("FullCommandLine", result.FullCommandLine);
+  xml.Element("StartTestTime", result.StartTestTime);
 }
 
 void cmCTestTestHandler::WriteTestResultFooter(cmXMLWriter& xml,

Source/CTest/cmCTestTestHandler.h

@@ -194,6 +194,7 @@ public:
     std::string TestMeasurementsOutput;
     std::string InstrumentationFile;
     int TestCount = 0;
+    std::chrono::milliseconds StartTestTime;
     cmCTestTestProperties* Properties = nullptr;
   };
 

Source/cmXMLWriter.h

@@ -113,6 +113,11 @@ private:
     return std::chrono::system_clock::to_time_t(value);
   }
 
+  static long long SafeContent(std::chrono::milliseconds value)
+  {
+    return value.count();
+  }
+
   template <typename T>
   static T SafeContent(T value)
   {