Use target dir that does not overlap with parent crate's

When reusing the original crate's target directory, crates compiled
during trybuild's invocation of Cargo are incompatible from the ones
compiled for the original test run but have colliding hashes on
compilers newer than nightly-2019-10-03, causing failures to find the
right crate during doc tests.

Examples of such failures:

```
   Doc-tests serde_json
error[E0463]: can't find crate for `serde_derive` which `serde` depends on
   --> /home/travis/build/serde-rs/json/src/lib.rs:325:1
    |
325 | extern crate serde;
    | ^^^^^^^^^^^^^^^^^^^ can't find crate
```

```
---- src/ext.rs - ext::TokenStreamExt::append_all (line 20) stdout ----
error[E0460]: found possibly newer version of crate `proc_macro2` which `quote` depends on
 --> src/ext.rs:19:1
  |
2 | extern crate quote;
  | ^^^^^^^^^^^^^^^^^^^
  |
  = note: perhaps that crate needs to be recompiled?
  = note: the following crate versions were found:
          crate `proc_macro2`: /home/travis/build/dtolnay/quote/target/debug/deps/libproc_macro2-e61df99964fc712a.rlib
          crate `proc_macro2`: /home/travis/build/dtolnay/quote/target/debug/deps/libproc_macro2-e61df99964fc712a.rmeta
          crate `proc_macro2`: /home/travis/build/dtolnay/quote/target/debug/deps/libproc_macro2-37a3c8079ede86c5.rmeta
          crate `quote`: /home/travis/build/dtolnay/quote/target/debug/deps/libquote-9e0852a9faf16dfb.rlib
```

src/cargo.rs

@@ -20,7 +20,7 @@ fn raw_cargo() -> Command {
 fn cargo(project: &Project) -> Command {
     let mut cmd = raw_cargo();
     cmd.current_dir(&project.dir);
-    cmd.env("CARGO_TARGET_DIR", &project.target_dir);
+    cmd.env("CARGO_TARGET_DIR", path!(project.target_dir / "tests" / "target"));
     rustflags::set_env(&mut cmd);
     cmd
 }