support --emit in 'test' command
support --emit in 'test' command

Support the `--emit` switch in `zig --emit asm test file.zig`.

The command fails because no tests run (no executable is created) but
it emits the requested file.  That seems like a good tradeoff.

Dimenus/zig,Dimenus/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,Dimenus/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig