fix(screenshot): alert user when toMatchScreenshot uses NaN (#4891)

ensures that `deviceScaleFactor` is properly narrowed to type `number`
from `number | undefined`. prior to this commit, the property _could_ be
set to `undefined`. this would result in multiplying by `undefined`,
producing `NaN` in our calculation of mismatched pixels. this value
would be used in a comparison operator, which would always return false.

that is, if `deviceScaleFactor` is `undefined`, we would always fail a
test using this matcher. that logic doesn't actually change - instead,
we throw a much more explicit error instead of silently failing

src/testing/jest/jest-27-and-under/matchers/screenshot.ts

@@ -12,7 +12,15 @@ export function toMatchScreenshot(compare: d.ScreenshotDiff, opts: d.MatchScreen
   }
 
   if (typeof compare.mismatchedPixels !== 'number') {
-    throw new Error(`expect toMatchScreenshot() value is not a screenshot compare`);
+    throw new Error(
+      `expect toMatchScreenshot() value is not a valid screenshot compare object - 'mismatchedPixels' has type '${typeof compare.mismatchedPixels}', but should be a number`,
+    );
+  }
+
+  if (typeof compare.deviceScaleFactor !== 'number') {
+    throw new Error(
+      `expect toMatchScreenshot() value is not a valid screenshot compare object - 'deviceScaleFactor' has type '${typeof compare.deviceScaleFactor}', but should be a number`,
+    );
   }
 
   const device = compare.device || compare.userAgent;