Remove most usage of target_platform

[gsnedders]

Currently we set target_platform based on what mozinfo returns for the given platform; the spec for platformName in an (informative) note suggests certain values, but it doesn't place any normative requirements on platformName, and thus these tests aren't justified by the spec.

Some of these can be rewritten to first create a session, and then use the value from that as the reference value; others seem impossible to write any test for.

See the individual tests. See also https://github.com/web-platform-tests/wpt/pull/52709 which does add some further coverage based on what the spec does require.