Upgrade to tracing 0.2.13

The primary motivation is to get the changes from
tokio-rs/tracing#990. Example output:

```
$ RUSTDOC_LOG=debug rustdoc +rustc2
warning: some trace filter directives would enable traces that are disabled statically
 | `debug` would enable the DEBUG level for all targets
 = note: the static max level is `info`
 = help: to enable DEBUG logging, remove the `max_level_info` feature
```

- Remove useless test

  This was testing for an ICE when passing `RUST_LOG=rustc_middle`.  I
  noticed it because it started giving the tracing warning (because tests
  are not run with debug-logging enabled). Since this bug seems unlikely
  to re-occur, I just removed it altogether.

Cargo.lock

@@ -5099,9 +5099,9 @@ dependencies = [
 
 [[package]]
 name = "tracing-core"
-version = "0.1.15"
+version = "0.1.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4f0e00789804e99b20f12bc7003ca416309d28a6f495d6af58d1e2c2842461b5"
+checksum = "f50de3927f93d202783f4513cda820ab47ef17f624b03c096e86ef00c67e6b5f"
 dependencies = [
  "lazy_static",
 ]
@@ -5129,9 +5129,9 @@ dependencies = [
 
 [[package]]
 name = "tracing-subscriber"
-version = "0.2.11"
+version = "0.2.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "abd165311cc4d7a555ad11cc77a37756df836182db0d81aac908c8184c584f40"
+checksum = "4ef0a5e15477aa303afbfac3a44cba9b6430fdaad52423b1e6c0dbbe28c3eedd"
 dependencies = [
  "ansi_term 0.12.1",
  "chrono",
@@ -5144,6 +5144,7 @@ dependencies = [
  "sharded-slab",
  "smallvec 1.4.2",
  "thread_local",
+ "tracing",
  "tracing-core",
  "tracing-log",
  "tracing-serde",

compiler/rustc_driver/Cargo.toml

@@ -10,7 +10,7 @@ crate-type = ["dylib"]
 [dependencies]
 libc = "0.2"
 tracing = { version = "0.1.18" }
-tracing-subscriber = { version = "0.2.10", default-features = false, features = ["fmt", "env-filter", "smallvec", "parking_lot", "ansi"] }
+tracing-subscriber = { version = "0.2.13", default-features = false, features = ["fmt", "env-filter", "smallvec", "parking_lot", "ansi"] }
 tracing-tree = "0.1.6"
 rustc_middle = { path = "../rustc_middle" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }