tests: Ensure stderr/stdout files are closed in run_js. NFC (#17544)

We say `test_dlmalloc_modes` failures on windows after #17512 which this
hopefully should address.

Author: sbc100

.circleci/config.yml

@@ -491,6 +491,7 @@ jobs:
             core2.test_em_asm_unicode
             core2.test_em_js
             core2.test_em_js_pthreads
+            other.test_dlmalloc_modes
             other.test_c_preprocessor
             other.test_prejs_unicode
             other.test_em_js_side_module

test/common.py

@@ -768,6 +768,7 @@ def run_js(self, filename, engine=None, args=None,
     else:
       stderr_file = self.in_dir('stderr')
       stderr = open(stderr_file, 'w')
+    stdout = open(stdout_file, 'w')
     error = None
     timeout_error = None
     if not engine:
@@ -778,13 +779,17 @@ def run_js(self, filename, engine=None, args=None,
       engine = engine + self.v8_args
     try:
       jsrun.run_js(filename, engine, args,
-                   stdout=open(stdout_file, 'w'),
+                   stdout=stdout,
                    stderr=stderr,
                    assert_returncode=assert_returncode)
     except subprocess.TimeoutExpired as e:
       timeout_error = e
     except subprocess.CalledProcessError as e:
       error = e
+    finally:
+      stdout.close()
+      if stderr != STDOUT:
+        stderr.close()
 
     # Make sure that we produced proper line endings to the .js file we are about to run.
     if not filename.endswith('.wasm'):