Disabled proxy unit test as it's succeptible to race condition

Cargo runs the test in multiple threads, but setting an environment
variable (here `HTTP_PROXY`) to a value _will_ break other tests that
assumes no variable were set.

Setting environment variable `RUST_TEST_THREAD` to 1 is not an option as
this would slow down the test phase.

src/gitlab.rs

@@ -320,23 +320,6 @@ mod tests {
                    debug);
     }
 
-    #[test]
-    fn gitlab_proxy() {
-        use std::env;
-
-        // NOTE: We cannot split this test in two, as tests are run in parallel their will be
-        // a race condition when setting the `HTTP_PROXY` environment variable.
-
-        // Use good proxy port. Should succeed.
-        env::set_var("HTTP_PROXY", "http://localhost:8000");
-        let _ = GitLab::new("gitlab.com", "XXXXXXXXXXXXXXXXXXXX").unwrap();
-
-        // Use bad proxy port. Constructor should fail
-        env::set_var("HTTP_PROXY", "http://localhost:abc");
-        let gl = GitLab::new("gitlab.com", "XXXXXXXXXXXXXXXXXXXX");
-        verify_err(&gl);
-    }
-
     #[test]
     fn gitlab_listers_groups() {
         let gl = GitLab::new("gitlab.com", "XXXXXXXXXXXXXXXXXXXX").unwrap();