test: fix unit tests failing on repeated runs (#39915)

`go test -count=2` failed in 12 packages because tests leaked state into
later runs.

- Reset SQLite sequences after each fixture load, like PostgreSQL
already does
- Prepare the database in tests that relied on earlier tests doing it
- Restore mutated globals with `test.MockVariableValue`

modules/templates/util_render_test.go

@@ -319,7 +319,7 @@ func TestRenderLabels(t *testing.T) {
 }
 
 func TestUserMention(t *testing.T) {
-	markup.RenderBehaviorForTesting.DisableAdditionalAttributes = true
+	defer test.MockVariableValue(&markup.RenderBehaviorForTesting.DisableAdditionalAttributes, true)()
 	rendered := newTestRenderUtils(t).MarkdownToHtml("@no-such-user @mention-user @mention-user")
 	assert.Equal(t, `<p>@no-such-user <a href="/mention-user" rel="nofollow">@mention-user</a> <a href="/mention-user" rel="nofollow">@mention-user</a></p>`, strings.TrimSpace(string(rendered)))
 }