test: improve coverage for tabsheet loading state fallback height

[web-padawan]

Description

Improved tests for the fallback height: current test was unreliable and didn't use any await.
This resolves following issues revealed when running Stryker mutation testing:

[Survived] StringLiteral
src/vaadin-tabsheet-mixin.js:223:35
-           content.style.minHeight = `${content.offsetHeight}px`;
+           content.style.minHeight = ``;

[Survived] BlockStatement
src/vaadin-tabsheet-mixin.js:221:38
-         } else if (hasOneVisiblePanel) {
-           // Make sure the empty content has a fallback height in loading state..
-           content.style.minHeight = `${content.offsetHeight}px`;
-         }
+         } else if (hasOneVisiblePanel) {}

[Survived] ConditionalExpression
src/vaadin-tabsheet-mixin.js:221:18
-         } else if (hasOneVisiblePanel) {
+         } else if (false) {

Type of change

• Test

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-review-bot]

✅ Nothing to flag — the changes look good.

[vaadin-bot]

This ticket/PR has been released with Vaadin 25.3.0-alpha6.