testserver: log error on "method not allowed" same as "not found" (#2933)

This extends the diagnostics we have for 404 to 405 errors.

Today, if you call testserver and there is no path registered, you'll
get 404 and a nice error message suggesting which setting to add to
config. If you call some path that is present but does not have handler
for the required method, you'll get 405 silently and without errors.

The deploy-artifact-path-type test had to be fixed since it relied on
undefined handler before and now it fails.

Author: denik

acceptance/bundle/telemetry/deploy-artifact-path-type/test.toml

@@ -13,3 +13,10 @@ Response.Body = '''
 [[Server]]
 Pattern = "PUT /api/2.0/fs/directories/Volumes/main/a/b/c/.internal"
 Response.StatusCode = 200
+
+[[Server]]
+Pattern = "GET /api/2.0/fs/directories/Volumes/main/a/b/c/.internal"
+Response.Body = '{}'
+# I'm adding 405 because that's what this test originally do. It's somewhat
+# surprising though that CLI can receive 405 and that does not result in error anywhere.
+Response.StatusCode = 405

libs/testserver/server.go

@@ -190,7 +190,7 @@ func New(t testutil.TestingT) *Server {
 	}
 
 	// Set up the not found handler as fallback
-	router.NotFoundHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	notFoundFunc := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		pattern := r.Method + " " + r.URL.Path
 		bodyBytes, err := io.ReadAll(r.Body)
 		var body string
@@ -227,6 +227,8 @@ Response.Body = '<response body here>'
 			t.Errorf("Response write error: %s", err)
 		}
 	})
+	router.NotFoundHandler = notFoundFunc
+	router.MethodNotAllowedHandler = notFoundFunc
 
 	return s
 }