Fix binary name in help message for test binaries
Currently the help output for a test binary uses the first argument instead of the binary name in the help output:
```
$ cargo test -- --help
...
Usage: --help [OPTIONS] [FILTERS...]
...
```
This fixes it to use the name of the binary (or `...` if there is no binary name passed on argv):
```
$ cargo test -- --help
...
Usage: /tmp/x/target/debug/deps/x-80c11a15ad4e1bf3 [OPTIONS] [FILTERS...]
...
```
76f968dadc