[test] Fix inconsistency regarding subprocess stdout/stderr buffering. NFC (#26087)

This bug could lead to tests that pass locally (when buffering is the
default) but not in CI (which doesn't do buffering by default).

Also, drive by fix to use existing EMCONFIGURE/EMCMAKE variables in
test_sockets.py.

Author: sbc100

test/common.py

@@ -1177,8 +1177,15 @@ def run_process(self, cmd, check=True, **kwargs):
 
     if stdout_buffering:
       sys.stdout.write(rtn.stdout)
+      # When we inject stdout/stderr buffering for our own internal purposes, we do not also want to
+      # make it available on the returned object.
+      # If we don't do this then callers would have access to rtn.stdout/rtn.stderr even when they
+      # didn't request it (i.e. even when they did not pass stderr=PIPE), which can lead to tests
+      # that pass in buffering mode, but fail without it.
+      rtn.stdout = None
     if stderr_buffering:
       sys.stderr.write(rtn.stderr)
+      rtn.stderr = None
     return rtn
 
   def emcc(self, filename, args=[], **kwargs):  # noqa

test/test_sockets.py

@@ -318,8 +318,8 @@ def test_enet(self):
     # this is also a good test of raw usage of emconfigure and emmake
     shutil.copytree(test_file('third_party', 'enet'), 'enet')
     with common.chdir('enet'):
-      self.run_process([path_from_root('emconfigure'), './configure', '--disable-shared'])
-      self.run_process([path_from_root('emmake'), 'make'])
+      self.run_process([common.EMCONFIGURE, './configure', '--disable-shared'])
+      self.run_process([common.EMMAKE, 'make'])
       enet = [self.in_dir('enet', '.libs', 'libenet.a'), '-I' + self.in_dir('enet', 'include')]
 
     with CompiledServerHarness(test_file('sockets/test_enet_server.c'), enet, 49210) as harness: