Set number of concurrent test threads to 1

Author: Stjepan Glavina

.travis.yml

@@ -16,4 +16,4 @@ script:
   - cargo fmt --all -- --check
   - cargo check --benches --bins --examples --tests
   - CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER="valgrind --leak-check=full --error-exitcode=1"
-    cargo test
+    cargo test -- --test-threads=1