Rework how E2E tests are executed

Tests kept breaking (not failing) randomly with "Broken pipe" errors.  Rework
how processes are spawned to make sure that it doesn't break them again.

test/e2e_test.py

@@ -20,6 +20,8 @@
 import os
 import sys
 
+import subprocess
+
 import difflib
 
 import configparser
@@ -70,7 +72,7 @@ def load_file(name):
 	contents = f.read()
 	f.close()
 
-	return contents
+	return bytes(contents, 'utf-8')
 
 def report_result(message_file, expected, test_output):
     status = None
@@ -91,23 +93,35 @@ def execute_e2e_test(case_name, config, config_path):
     # This environment variable is set in Makefile.
     python_path = os.getenv('PYTHON', 'python3')
 
-    test_command = "GPG_MAILGATE_CONFIG=%s %s gpg-mailgate.py %s < %s" % (
-        config_path,
-        python_path,
-        config.get(case_name, "to"),
-        config.get(case_name, "in"))
-    result_command = "%s %s %d" % (python_path, config.get("relay", "script"), config.getint("relay", "port"))
+    gpglacre_cmd = [python_path,
+                    "gpg-mailgate.py",
+                    config.get(case_name, "to")]
 
-    logging.debug("Spawning relay: '%s'" % (result_command))
-    pipe = os.popen(result_command, 'r')
+    relay_cmd = [python_path,
+                 config.get("relay", "script"),
+                 config.get("relay", "port")]
 
-    logging.debug("Spawning GPG-Lacre: '%s'" % (test_command))
-    msgin = os.popen(test_command, 'w')
-    msgin.write(load_file(config.get(case_name, "in")))
-    msgin.close()
+    logging.debug("Spawning relay: '%s'" % (relay_cmd))
+    relay_proc = subprocess.Popen(relay_cmd,
+                                  stdin = None,
+                                  stdout = subprocess.PIPE)
 
-    testout = pipe.read()
-    pipe.close()
+    logging.debug("Spawning GPG-Lacre: '%s', stdin = %s"
+                  % (gpglacre_cmd,
+                     config.get(case_name, "in")))
+
+    # pass PATH because otherwise it would be dropped
+    gpglacre_proc = subprocess.run(gpglacre_cmd,
+                                   input = load_file(config.get(case_name, "in")),
+                                   capture_output = True,
+                                   env = {"GPG_MAILGATE_CONFIG": config_path,
+                                          "PATH": os.getenv("PATH")})
+
+    # Let the relay process the data.
+    relay_proc.wait()
+
+    (testout, _) = relay_proc.communicate()
+    testout = testout.decode('utf-8')
 
     logging.debug("Read %d characters of test output: '%s'" % (len(testout), testout))