Fix parallel race condition in tests emrun.test_emrun and emrun.test_emrun_pthreads (#26790)

Fix parallel race condition in tests emrun.test_emrun and
emrun.test_emrun_pthreads, which would not be able to run
simultaneously, since they attempted to launch emrun server on the same
port. Each parallel process needs its own server port, but different
individual test cases do not.

Author: juj

test/test_browser.py

@@ -5765,8 +5765,14 @@ def test_emrun(self):
     # delete it. Therefore switch away from that directory before launching.
     os.chdir(path_from_root())
 
+    # emrun tests may run in parallel processes, so each test case should use a unique port number
+    # to avoid port address already in use errors.
+    port = '6939'
+    if '-pthread' in self.cflags:
+      port = '6940'
+
     args_base = [EMRUN, '--timeout', '30', '--safe_firefox_profile',
-                 '--kill-exit', '--port', '6939', '--verbose',
+                 '--kill-exit', '--port', port, '--verbose',
                  '--log-stdout', self.in_dir('stdout.txt'),
                  '--log-stderr', self.in_dir('stderr.txt')]
 
@@ -5791,9 +5797,9 @@ def test_emrun(self):
     for args in [
         [],
         ['--port', '0'],
-        ['--private_browsing', '--port', '6941'],
-        ['--dump_out_directory', 'other dir/multiple', '--port', '6942'],
-        ['--dump_out_directory=foo_bar', '--port', '6942'],
+        ['--private_browsing'],
+        ['--dump_out_directory', 'other dir/multiple'],
+        ['--dump_out_directory=foo_bar'],
     ]:
       args = args_base + args + [self.in_dir('test_emrun.html'), '--', '1', '2', '--3', 'escaped space', 'with_underscore']
       print(shlex.join(args))