stress-test: disable CSS transitions on injected styles (microsoft#25559

)

This addresses an issue where Firefox's style recalculation measurement
becomes unstable when controls under test have CSS transitions.

Specifically, our test injects styles that update the background color
of selected controls and many Fluent controls transition the background
color for different states like "hover" and "focus".

With transitions running Firefox will sometimes include the transition
time in the style recalc measurement and sometimes not. Additionally,
Chromium browsers never include the transition time in the measurement.

Disabling transitions ensures measurement behavior is consistent in
Firefox and in alignment with the behavior of Chromium browsers.

apps/stress-test/src/shared/css/injectStyles.ts

@@ -158,7 +158,7 @@ export const randomCssFromSelectors = (selectors: string[]): string => {
   let css = '';
 
   selectors.forEach(selector => {
-    css += `${selector} { background-color: ${choice(colors)}; }\n`;
+    css += `${selector} { background-color: ${choice(colors)}; transition-duration: 0ms; }\n`;
   });
 
   return css;