tests: rename testcommon module to shared_test_code

This name always confuses me, since there is also test_common.py. And this
module is not actually a test suite, even though it starts with "test".
This also makes for better tab completion, e.g.

python3 -m unittest tests/te[Tab] -> tests/test_

tests/shared_test_code.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2017, Michael Poehn <michael.poehn@fsfe.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import os
+import sys
+import tempfile
+import unittest
+import unittest.mock
+
+from pathlib import Path
+
+
+GP_FINGERPRINT = 'B7C2EEFD8DAC7806AF67DFCD92EB18126BC08312A7F2D6F3862E46013C7A6135'
+
+
+class VerboseFalseOptions:
+    verbose = False
+
+
+class TmpCwd:
+    """Context-manager for temporarily changing the current working directory."""
+
+    def __init__(self, new_cwd):
+        self.new_cwd = new_cwd
+
+    def __enter__(self):
+        self.orig_cwd = os.getcwd()
+        os.chdir(self.new_cwd)
+
+    def __exit__(self, a, b, c):
+        os.chdir(self.orig_cwd)
+
+
+class TmpPyPath:
+    """Context-manager for temporarily adding a directory to Python path."""
+
+    def __init__(self, additional_path):
+        self.additional_path = additional_path
+
+    def __enter__(self):
+        sys.path.append(self.additional_path)
+
+    def __exit__(self, a, b, c):
+        sys.path.remove(self.additional_path)
+
+
+def mock_open_to_str(mock):
+    """For accessing all data written into a unittest.mock.mock_open() instance as a string."""
+
+    return "".join(
+        [x.args[0] for x in mock.mock_calls if str(x).startswith("call().write(")]
+    )
+
+
+def mkdtemp():
+    if sys.version_info < (3, 10):  # ignore_cleanup_errors was added in 3.10
+        return tempfile.TemporaryDirectory()
+    else:
+        return tempfile.TemporaryDirectory(ignore_cleanup_errors=True)
+
+
+def mkdir_testfiles(localmodule, test):
+    """Keep the test files in a labeled test dir for easy reference"""
+    testroot = Path(localmodule) / '.testfiles'
+    testroot.mkdir(exist_ok=True)
+    testdir = testroot / unittest.TestCase.id(test)
+    testdir.mkdir(exist_ok=True)
+    return tempfile.mkdtemp(dir=testdir)
+
+
+def mock_urlopen(status=200, body=None):
+    resp = unittest.mock.MagicMock()
+    resp.getcode.return_value = status
+    resp.read.return_value = body
+    resp.__enter__.return_value = resp
+    return unittest.mock.Mock(return_value=resp)