WASM: Browser Selection in Test Helpers is broken

https://codereview.qt-project.org/c/qt/qtbase/+/587336 added chrome browser detection to QTestHelpers.cmake. It expects the environment variable BROWSER_FOR_WASM to be set at configure time.

In an CI environment, this is not the case at configure time, see here.

The browser command therefore expands to chrome, which at that point is not in PATH.

Critical, because it renders webassembly-linux-thread-asyncify-tests dysfunctional.