update tests to reflect changes in Lightning CSS

This commit (parcel-bundler/lightningcss@284debf) introduced:

"Don't put unknown pseudo elements in an `:is()`"

I thought this was a bug at first, but looking at MDN, these pseudo
elements are non-standard:

- https://developer.mozilla.org/en-US/docs/Web/CSS/::-webkit-inner-spin-button
- https://developer.mozilla.org/en-US/docs/Web/CSS/::-webkit-outer-spin-button

Author: RobinMalfait

tests/plugins/__snapshots__/preflight.test.js.snap

@@ -115,7 +115,11 @@ progress {
   vertical-align: baseline;
 }
 
-:is(::-webkit-inner-spin-button, ::-webkit-outer-spin-button) {
+::-webkit-inner-spin-button {
+  height: auto;
+}
+
+::-webkit-outer-spin-button {
   height: auto;
 }
 
@@ -308,7 +312,11 @@ progress {
   vertical-align: baseline;
 }
 
-:is(::-webkit-inner-spin-button, ::-webkit-outer-spin-button) {
+::-webkit-inner-spin-button {
+  height: auto;
+}
+
+::-webkit-outer-spin-button {
   height: auto;
 }