Warn when a test has two @only-target?

[RalfJung]

I just realized that we recently accidentally disabled a test in Miri... I wonder if we should warn against having multiple @only-target?

OTOH that could be legitimate when it's something like

// Only on linux on x86 (32/64) targets
@only-target: linux
@only-target: 86

[oli-obk]

We could allow regexes in the pattern and then forbid there being multiple only-target or only-host