Fix flaky TestPlugin (#13933)

- Fix panic due to not checking for JSON marshal error
- Reorder the FileExists check which would fail if we were to pass
the -short flag to bypass the slow URL test. In which case, the plugin
would have been removed from the filesystem.
So we move the check before we remove the plugin.

https://mattermost.atlassian.net/browse/MM-22666

api4/plugin_test.go

@@ -33,7 +33,8 @@ func TestPlugin(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 
-	statesJson, _ := json.Marshal(th.App.Config().PluginSettings.PluginStates)
+	statesJson, err := json.Marshal(th.App.Config().PluginSettings.PluginStates)
+	require.Nil(t, err)
 	states := map[string]*model.PluginState{}
 	json.Unmarshal(statesJson, &states)
 	th.App.UpdateConfig(func(cfg *model.Config) {
@@ -66,6 +67,11 @@ func TestPlugin(t *testing.T) {
 	CheckNoError(t, resp)
 	assert.Equal(t, "testplugin", manifest.Id)
 
+	// Stored in File Store: Install Plugin from URL case
+	pluginStored, err := th.App.FileExists("./plugins/" + manifest.Id + ".tar.gz")
+	assert.Nil(t, err)
+	assert.True(t, pluginStored)
+
 	ok, resp := th.SystemAdminClient.RemovePlugin(manifest.Id)
 	CheckNoError(t, resp)
 	require.True(t, ok)
@@ -88,11 +94,6 @@ func TestPlugin(t *testing.T) {
 		assert.Equal(t, "testplugin", manifest.Id)
 	})
 
-	// Stored in File Store: Install Plugin from URL case
-	pluginStored, err := th.App.FileExists("./plugins/" + manifest.Id + ".tar.gz")
-	assert.Nil(t, err)
-	assert.True(t, pluginStored)
-
 	th.App.RemovePlugin(manifest.Id)
 
 	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.PluginSettings.Enable = false })