Fix a PyLint 2.13.7 error

PyLint 2.13.7 reports the following error:

    bin/tests/system/doth/conftest.py:34:28: E0601: Using variable 'stderr' before assignment (used-before-assignment)

The reason the current code has not caused problems before is that
invoking gnutls-cli with just the --logfile=/dev/null argument causes it
to always return with a non-zero exit code, either due to the option not
being supported or due to the hostname argument not being provided.  In
other words, the 'except' branch has always been taken.  PyLint is
obviously right on a syntactical level, though.

Instead of relying on a less than obvious code flow (where the 'except'
branch is always taken), rework the flagged code by employing
subprocess.run(..., check=False) instead of subprocess.check_output(),
making exception handling redundant.

While this issue was investigated, it was also noticed that
subprocess.check_output() was incorrectly used as a context manager:
Popen objects are context managers, but subprocess.check_output() and
subprocess.run() are not.  Fix by dropping the relevant 'with'
statement.

bin/tests/system/doth/conftest.py

@@ -25,13 +25,10 @@ def gnutls_cli_executable():
         pytest.skip('gnutls-cli not found in PATH')
 
     # Ensure gnutls-cli supports the --logfile command-line option.
-    args = [executable, '--logfile=/dev/null']
-    try:
-        with subprocess.check_output(args, stderr=subprocess.STDOUT) as _:
-            pass
-    except subprocess.CalledProcessError as exc:
-        stderr = exc.output
-    if b'illegal option' in stderr:
+    output = subprocess.run([executable, '--logfile=/dev/null'],
+                            stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
+                            check=False).stdout
+    if b'illegal option' in output:
         pytest.skip('gnutls-cli does not support the --logfile option')
 
     return executable