ctest: Convert StartTestTime field in Test.xml to seconds

In commit fd55647d16 (ctest: Record `StartTestTime` entry for each test
in Test.xml, 2026-02-12) we added the field with milliseconds to match
instrumentation.  However, all other times reported by CTest are in
seconds, including the global `StartTestTime` field.  Preserve
sub-second resolution using a floating-point representation.

Issue: #26859

Source/CTest/cmCTestRunTest.cxx

@@ -366,8 +366,7 @@ cmCTestRunTest::EndTestResult cmCTestRunTest::EndTest(size_t completed,
       this->TestResult.CompletionStatus = "Completed";
     }
     this->TestResult.StartTestTime =
-      std::chrono::duration_cast<std::chrono::milliseconds>(
-        this->TestProcess->GetSystemStartTime().time_since_epoch());
+      this->TestProcess->GetSystemStartTime().time_since_epoch();
     this->TestResult.ExecutionTime = this->TestProcess->GetTotalTime();
     this->MemCheckPostProcess();
     this->ComputeWeightedCost();

Source/CTest/cmCTestTestHandler.cxx

@@ -652,6 +652,12 @@ bool cmCTestTestHandler::GenerateXML()
       this->LogFile = nullptr;
       return false;
     }
+
+    // We represent some times as a double-precision floating-point number
+    // of seconds since the epoch.  Print them with microsecond precision.
+    // Representable values differ by hundreds of nanoseconds anyway.
+    xmlfile << std::fixed << std::setprecision(6);
+
     cmXMLWriter xml(xmlfile);
     this->GenerateCTestXML(xml);
   }

Source/CTest/cmCTestTestHandler.h

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

Source/cmXMLWriter.h

@@ -12,6 +12,7 @@
 #include <string>
 #include <vector>
 
+#include "cmDuration.h"
 #include "cmXMLSafe.h"
 
 class cmXMLWriter
@@ -113,10 +114,8 @@ private:
     return std::chrono::system_clock::to_time_t(value);
   }
 
-  static long long SafeContent(std::chrono::milliseconds value)
-  {
-    return value.count();
-  }
+  /* Some code paths use time_since_epoch despite the unspecified epoch.  */
+  static double SafeContent(cmDuration value) { return value.count(); }
 
   template <typename T>
   static T SafeContent(T value)