Run test in serial by default to not take all memory

When running all the test in parallel, all the memory on my laptop
is consumed by rustc processes, my machine become unusable and the
tests do not make progress.

It seems to make sense to have serial by default, as the number of
process depends on the number of test files.

To run the test in parallel:
RUN_TEST_PARALLEL=1 cargo test --feature llvm_stable

Author: jeanphilippeD

tests/tests.rs

@@ -85,24 +85,31 @@ fn run_bindgen_tests() {
         }
     });
 
-    // First spawn all child processes and collect them, then wait on each
-    // one. This runs the tests in parallel rather than serially.
-
-    let children: Vec<_> = tests.map(|entry| {
+    let children = tests.map(|entry| {
             let child = spawn_run_bindgen(run_bindgen.clone(), bindgen.clone(), entry.path());
             (entry.path(), child)
-        })
-        .collect();
-
-    let failures: Vec<_> = children.into_iter()
-        .filter_map(|(path, mut child)| {
-            let passed = child.wait()
-                .expect("Should wait on child process")
-                .success();
-
-            if passed { None } else { Some((path, child)) }
-        })
-        .collect();
+        });
+
+    let child_wait = |(path, mut child): (PathBuf, process::Child)| {
+        let passed = child.wait()
+            .expect("Should wait on child process")
+            .success();
+
+        if passed { None } else { Some((path, child)) }
+    };
+
+    let failures: Vec<_> = if env::var("RUN_TEST_PARALLEL").is_ok() {
+        // First spawn all child processes and collect them,
+        // then wait on each one.
+        children.collect::<Vec<_>>().into_iter()
+        .filter_map(child_wait)
+        .collect()
+    } else {
+        // Spawn one child at a time and wait on it.
+        children
+        .filter_map(child_wait)
+        .collect()        
+    };
 
     let num_failures = failures.len();