Revamp "./miri test": parse --target ourselves, pass the rest to the test harness

[RalfJung]

./miri test <flags> just sends all the flags to cargo as-is. That's suboptimal for (at least) two reasons:

• ./miri test filter works, but to filter two things one has to do ./miri test -- filter1 filter2. This is also how cargo works but it is surprising.
• ./miri test --target foo interprets foo as a host target. That's consistent with ./miri build, but it's basically never what you want. MIRI_TEST_TARGET=foo ./miri test is what you want, but that's longer to type, non-standard, and especially inconvenient on Windows.

So I think we should make this command be ./miri test [--target target] <flags>, where target does the same thing as MIRI_TEST_TARGET, and <flags> are passed to the test harness -- i.e., the invocation will be cargo test -- <flags> (note the extra --).

[RalfJung]

@RossSmyth would you like to give this a try? :)

[RossSmyth]

#3588

Don't worry, I planned on working on it :)

[RalfJung]

Fixes by #3588