Answer architecture round 1: a custom CLI names itself by its file, --version names its module beside fejkdata's and follows a replace, the module path comes from the library's type, the end-to-end test runs main in a subprocess, the CLI tests find the shipped modules by their manifests, and Your own CLI sits under Library

cmd/fejkdata/main_test.go

@@ -1,32 +1,28 @@
 package main
 
 import (
-	"io"
+	"bytes"
 	"os"
+	"os/exec"
+	"strings"
 	"testing"
-
-	"github.com/larvit/fejkdata/cli"
-	"github.com/larvit/fejkdata/data"
 )
 
+func TestMain(m *testing.M) {
+	if os.Getenv("FEJKDATA_RUN_MAIN") == "1" {
+		main()
+	}
+	os.Exit(m.Run())
+}
+
 func TestMainRendersTheShippedData(t *testing.T) {
-	stdin, feed, err := os.Pipe()
-	if err != nil {
-		t.Fatal(err)
-	}
-	drain, stdout, err := os.Pipe()
-	if err != nil {
-		t.Fatal(err)
-	}
-	args, oldIn, oldOut := os.Args, os.Stdin, os.Stdout
-	t.Cleanup(func() { os.Args, os.Stdin, os.Stdout = args, oldIn, oldOut })
-	os.Args, os.Stdin, os.Stdout = []string{"fejkdata", "--seed", "1"}, stdin, stdout
-	io.WriteString(feed, "{/misc.territory[SE].alpha2} {/en_US.person.last} {/sv_SE.address.locality}")
-	feed.Close()
-	code := cli.Main(data.Modules()...)
-	stdout.Close()
-	out, _ := io.ReadAll(drain)
-	if code != 0 || len(out) < len("SE x y") || string(out[:3]) != "SE " {
-		t.Errorf("fejkdata = %d, %q, want SE followed by a last name and a locality", code, out)
+	cmd := exec.Command(os.Args[0], "--seed", "1")
+	cmd.Env = append(os.Environ(), "FEJKDATA_RUN_MAIN=1")
+	cmd.Stdin = strings.NewReader("{/misc.territory[SE].alpha2} {/en_US.person.last} {/sv_SE.address.locality}")
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	out, err := cmd.Output()
+	if fields := strings.Fields(string(out)); err != nil || len(fields) < 3 || fields[0] != "SE" {
+		t.Errorf("fejkdata = %v, %q, %q, want SE followed by a last name and a locality", err, out, stderr.String())
 	}
 }