Warn on conditional value usage in `tags` properties for Firefox

[ErichDonGubler]

tags properties are intended to be used as a facility for filtering tests to run in invocations to wptrunner via the --tag and --exclude-tag flags.

However, in Mozilla CI specifically, tags should avoid conditional values, because tags are used as a way to filter the test paths distributed to CI tasks of wptrunner before they are executed on runner machines. It's not generally possible to evaluate conditional values' clauses in that context, because the clause evaluation environment of each platform and build configuration is not known in advance of this distribution. I'm not sure what the consequence of doing so, but I imagine it results in such a property being ignored. So:

When all the following are true:

• tags properties are discovered in metadata when moz-webgpu-cts subcommand execute
• --browser firefox is set (the default)

…then we should emit a WARN-level log indicating that the user of this risk, and encourage them to remove the conditional clauses.

[ErichDonGubler]

CC'ing @jgraham to confirm understanding here.