Improve the test for input from stdin

The test would not fail without the change because it was not making any
assertions about the input. Adding a check for "DONE 1" causes it to
fail properly when the support is removed.

Also add an example to the docs of how to use stdin.

README.md

@@ -232,6 +232,11 @@ stdout and stderr output:
   stderr, not the `test2json` stdout). Any stderr produced by tests is not
   considered an error (it will be in the `test2json` stdout).
 
+**Example: accept intput from stdin
+```
+cat out.json | gotestsum --raw-command -- cat
+```
+
 **Example: run tests with profiling enabled**
 
 Using a `profile.sh` script like this:

cmd/main_test.go

@@ -7,9 +7,10 @@ import (
 	"os/exec"
 	"strings"
 	"testing"
-	"time"
 
+	"gotest.tools/gotestsum/testjson"
 	"gotest.tools/v3/assert"
+	"gotest.tools/v3/assert/cmp"
 	"gotest.tools/v3/env"
 	"gotest.tools/v3/golden"
 )
@@ -402,43 +403,40 @@ func TestRun_RerunFails_PanicPreventsRerun(t *testing.T) {
 	assert.ErrorContains(t, err, "rerun aborted because previous run had a suspected panic", out.String())
 }
 
-func TestExecStdin(t *testing.T) {
+func TestRun_InputFromStdin(t *testing.T) {
 	stdin := os.Stdin
 	t.Cleanup(func() { os.Stdin = stdin })
 
 	r, w, err := os.Pipe()
 	assert.NilError(t, err)
-	defer r.Close()
+	t.Cleanup(func() { _ = r.Close() })
 
 	os.Stdin = r
 
 	go func() {
-		defer w.Close()
+		defer func() { _ = w.Close() }()
+
 		e := json.NewEncoder(w)
-		for _, event := range []struct {
-			Time    *time.Time `json:",omitempty"`
-			Action  string
-			Package string  `json:",omitempty"`
-			Test    string  `json:",omitempty"`
-			Elapsed float64 `json:",omitempty"`
-			Output  string  `json:",omitempty"`
-		}{
+		for _, event := range []testjson.TestEvent{
 			{Action: "run", Package: "pkg"},
 			{Action: "run", Package: "pkg", Test: "TestOne"},
 			{Action: "fail", Package: "pkg", Test: "TestOne"},
 			{Action: "fail", Package: "pkg"},
 		} {
-			assert.NilError(t, e.Encode(event))
+			assert.Check(t, e.Encode(event))
 		}
 	}()
 
-	assert.NilError(t, run(&options{
+	stdout := new(bytes.Buffer)
+	err = run(&options{
 		args:        []string{"cat"},
 		format:      "testname",
 		hideSummary: newHideSummaryValue(),
 		rawCommand:  true,
 
-		stdout: os.Stdout,
+		stdout: stdout,
 		stderr: os.Stderr,
-	}))
+	})
+	assert.NilError(t, err)
+	assert.Assert(t, cmp.Contains(stdout.String(), "DONE 1"))
 }