Revert "CI: Temporarily run tests with only one thread"

This reverts commit 24c0ca7d42.

That commit is no longer necessary after 10ed46d.

.travis.yml

@@ -7,9 +7,7 @@ matrix:
   # tests pass
   - rust: nightly
     script:
-    # XXX: `--test-threads 1` is a hack until we have a proper fix for
-    # synchronizing access to the fixtures.
-    - cargo test --locked -- --test-threads 1
+    - cargo test --locked
     - rustup component add rustfmt-preview
     - cargo fmt --all -- --check
     env: RUST_BACKTRACE=1