Fix "test" subcommand's pass-through arguments

[cpick]

The cargo-zigbuild "test" subcommand has an optional, trailing positional TESTNAME argument. This arg sometimes contains a test name filter and sometimes just another flag that should be passed through to the underlying test binary.

For example these work fine (even if args has unexpected contents):

1. cargo-zigbuild test or cargo-zigbuild test --: test_name: None args: []
2. cargo-zigbuild test foo or cargo-zigbuild test -- foo: test_name: Some("foo") args: []
3. cargo-zigbuild test foo bar baz, cargo-zigbuild test -- foo bar baz, or cargo-zigbuild test foo -- bar baz: test_name: Some("foo") args: ["bar", "baz"]

However this does not work (as a result of an unexpected test_name):
4. cargo-zigbuild test -- --nocapture: test_name: Some("--nocapture") args: []

This is becaue cargo-zigbuild test will pass --nocapture through to cargo test without prefixing it with a -- separator (since it mistakenly treats it as a test name filter, not an additional argument).

Likewise the following fails for similar reasons:
5. cargo-zigbuild test -- --nocapture --include-ignored: test_name: Some("--nocapture") args: ["--include-ignored"]

But the following works (since it includes a non-arg test_name):
6. cargo-zigbuild test -- foo --nocapture --include-ignored or cargo-zigbuild test foo -- --nocapture --include-ignored: test_name: Some("foo") args: ["--nocapture", "--include-ignored"]

In all of the above, the cargo "test" subcommand will do the right thing if a -- separator is placed before any test name and/or additional arguments (and also if neither are present). So always pass the separator before either of those.

Fixes: rust-cross/cargo-zigbuild#240