Run the internal/handlers tests in parallel (closes #194)

The handlers tests ran one at a time because config.New writes the
global viper instance. Each test spends most of its time in SQLite
under the race detector, so run in a row they took longer than the
20-second target.

Every test and subtest now calls t.Parallel(), as the tests in the
other server packages already do. A mutex in newTestServerWith makes
tests start their servers one at a time, since config.New writes
viper and the server reads it as it starts. No test is removed or
otherwise changed.

Model: opus-5-5

internal/handlers/utility_test.go

@@ -1,4 +1,3 @@
-//nolint:paralleltest // global viper, as in api_test.go
 package handlers_test
 
 import (
@@ -72,6 +71,8 @@ func numericBody(
 }
 
 func TestUserhost(t *testing.T) {
+	t.Parallel()
+
 	tserver := newTestServer(t)
 
 	token := tserver.createSession("alice")
@@ -109,6 +110,8 @@ func TestUserhost(t *testing.T) {
 }
 
 func TestVersionAdminInfoTime(t *testing.T) {
+	t.Parallel()
+
 	tserver := newTestServer(t)
 
 	token := tserver.createSession("frank")
@@ -147,6 +150,8 @@ func TestVersionAdminInfoTime(t *testing.T) {
 }
 
 func TestKill(t *testing.T) {
+	t.Parallel()
+
 	tserver := newTestServerWithOper(t)
 
 	victimToken := tserver.createSession("victim")
@@ -213,6 +218,8 @@ func TestKill(t *testing.T) {
 }
 
 func TestWallops(t *testing.T) {
+	t.Parallel()
+
 	tserver := newTestServerWithOper(t)
 
 	receiverToken := tserver.createSession("receiver")
@@ -258,6 +265,8 @@ func TestWallops(t *testing.T) {
 }
 
 func TestUserMode(t *testing.T) {
+	t.Parallel()
+
 	const (
 		nick  = "alice"
 		other = "other"