go junitxml: compute per package start times rather than using per execution

internal/junitxml/report.go

@@ -104,11 +104,23 @@ func generate(exec *testjson.Execution, cfg Config) JUnitTestSuites {
 	if cfg.customElapsed != "" {
 		suites.Time = cfg.customElapsed
 	}
+
+	overallEarliestTimestamp := exec.EarliestTime()
+	if overallEarliestTimestamp.IsZero() {
+		overallEarliestTimestamp = exec.Started()
+	}
+
 	for _, pkgname := range exec.Packages() {
 		pkg := exec.Package(pkgname)
 		if cfg.HideEmptyPackages && pkg.IsEmpty() {
 			continue
 		}
+
+		earliestTimestamp := pkg.EarliestTime()
+		if earliestTimestamp.IsZero() {
+			earliestTimestamp = overallEarliestTimestamp
+		}
+
 		junitpkg := JUnitTestSuite{
 			Name:       cfg.FormatTestSuiteName(pkgname),
 			Tests:      pkg.Total,
@@ -119,7 +131,7 @@ func generate(exec *testjson.Execution, cfg Config) JUnitTestSuites {
 			Timestamp:  cfg.customTimestamp,
 		}
 		if cfg.customTimestamp == "" {
-			junitpkg.Timestamp = exec.Started().Format(time.RFC3339)
+			junitpkg.Timestamp = earliestTimestamp.Format(time.RFC3339)
 		}
 		suites.Suites = append(suites.Suites, junitpkg)
 	}

testjson/execution.go

@@ -131,6 +131,21 @@ func (p *Package) TestCases() []TestCase {
 	return tc
 }
 
+// EarliestTime returns the earliest start time found within a child test case.
+func (p *Package) EarliestTime() time.Time {
+	found := false
+	earliest := time.Time{}
+
+	for _, tc := range p.TestCases() {
+		if !found || tc.Time.Before(earliest) {
+			earliest = tc.Time
+			found = true
+		}
+	}
+
+	return earliest
+}
+
 // LastFailedByName returns the most recent test with name in the list of Failed
 // tests. If no TestCase is found with that name, an empty TestCase is returned.
 //
@@ -652,6 +667,26 @@ func (e *Execution) Started() time.Time {
 	return e.started
 }
 
+// EarliestTime returns the earliest runtime found in any of the packages within this execution.
+func (e *Execution) EarliestTime() time.Time {
+	found := false
+	earliest := time.Time{}
+
+	for _, p := range e.packages {
+		candidate := p.EarliestTime()
+		if candidate.IsZero() {
+			continue
+		}
+
+		if !found || candidate.Before(earliest) {
+			earliest = candidate
+			found = true
+		}
+	}
+
+	return earliest
+}
+
 // newExecution returns a new Execution and records the current time as the
 // time the test execution started.
 func newExecution() *Execution {