Auto merge of #1244 - Xanewok:translate-tests, r=alexheretic

Translate cmd tests to use async LSP client

This translates our cmd-based tests to use the LSP client introduced in #1223.

Some anecdotal benchmarks (Xubuntu 18.04, Ryzen 2600) before and after translating:
```
$ time cargo test cmd_
real    0m3,498s
user    0m8,379s
sys     0m2,710s

$ time cargo test cmd_ --release
real    0m3,059s
user    0m5,585s
sys     0m2,512s

$ time cargo test client_
real    0m3,465s
user    0m7,197s
sys     0m1,926s

$ time cargo test client_ --release
real    0m3,053s
user    0m4,977s
sys     0m1,825s
```
It seems that synchronization overhead caused by running multiple RLS and rustc instances in-process still outweighs spawning separate processes per test 🎉 That means we don't lose performance (and actually benefit) from switching over.

I plan on translating the remaining tests in tests_old as a separate PR.

r? @alexheretic

Cargo.toml

@@ -61,7 +61,7 @@ rustc-workspace-hack = "1.0.0"
 [dev-dependencies]
 difference = "2"
 tempfile = "3"
-lsp-codec = "0.1"
+lsp-codec = "0.1.1"
 tokio = "0.1"
 futures = "0.1"
 tokio-process = "0.2"