pytest: close file handles after use, and two minor tidy-ups

Also:
- drop two unreachable return statements.
- test_17_ssl_use: avoid implicit string concatenations in lists.

Reported by GitHub CodeQL

Closes #21916

tests/http/testenv/client.py

@@ -104,8 +104,9 @@ class LocalClient:
             log.warning(f'Timeout after {self._timeout}s: {args}')
             exitcode = -1
             exception = 'TimeoutExpired'
-        coutput = open(self._stdoutfile).readlines()
-        cerrput = open(self._stderrfile).readlines()
+        with open(self._stdoutfile) as fout, open(self._stderrfile) as ferr:
+            coutput = fout.readlines()
+            cerrput = ferr.readlines()
         return ExecResult(args=myargs, exit_code=exitcode, exception=exception,
                           stdout=coutput, stderr=cerrput,
                           duration=datetime.now() - start)
@@ -113,8 +114,10 @@ class LocalClient:
     def dump_logs(self):
         lines = []
         lines.append('>>--stdout ----------------------------------------------\n')
-        lines.extend(open(self._stdoutfile).readlines())
+        with open(self._stdoutfile) as cstdout:
+            lines.extend(cstdout.readlines())
         lines.append('>>--stderr ----------------------------------------------\n')
-        lines.extend(open(self._stderrfile).readlines())
+        with open(self._stderrfile) as cstderr:
+            lines.extend(cstderr.readlines())
         lines.append('<<-------------------------------------------------------\n')
         return ''.join(lines)