This should fix https://github.com/flutter/flutter/issues/126178

When we don't pass a `--concurrency` flag to the test package, it uses a default based on the number of cores that are on the machine. However, the web test platform itself serializes all these requests anyway, which can lead to the test package timing out. This is because from the test package's perspective, it has already started the loading process on a number of suites which are simply waiting for other test suites to compile and run. The ones that wait the longest can run up against the test packages 12 minute timeout for loading a given suite, even though they haven't actually started to try to load.

Instead, we should always pass `--concurrency=1` to the test package so that it doesn't attempt to start loads concurrently in the first place.