mir-json-rustc-wrapper produced linked MIR with too many dots in the file name

[RyanGlScott]

Because of this line:

mir-json/src/bin/mir-json-rustc-wrapper.rs

Line 255 in fbacd2a

let json_path = test_path.with_extension(".linked-mir.json");

Calling mir-json-rustc-wrapper on hello.rs will produce hello..linked-mir.json (notice the two consecutive dots). I think we should change the line above to with_extension("linked-mir.json"); to avoid this.