[test] Add cmdline tests for the diff feature

We usually test CodeChecker diff by making API calls that one can find
in report_server.py. However, `CodeChecker cmd diff` doesn't invoke
these plainly, but rather wraps it in various functions, for which we
never had proper tests. Until now!

I also made sure to include (almost all) cases discussed here:
Ericsson#3884

web/server/codechecker_server/api/mass_store_run.py

@@ -1323,10 +1323,15 @@ def store(self) -> int:
                         runtime = round(time.time() - start_time, 2)
                         zip_size_kb = round(zip_size / 1024)
 
+                        tag_desc = ""
+                        if self.__tag:
+                            tag_desc = f", under tag '{self.__tag}'"
+
                         LOG.info("'%s' stored results (%s KB "
-                                 "/decompressed/) to run '%s' (id: %d) in "
+                                 "/decompressed/) to run '%s' (id: %d) %s in "
                                  "%s seconds.", self.user_name,
-                                 zip_size_kb, self.__name, run_id, runtime)
+                                 zip_size_kb, self.__name, run_id, tag_desc,
+                                 runtime)
 
                         iso_start_time = datetime.fromtimestamp(
                             start_time).isoformat()

web/tests/functional/__init__.py

@@ -23,4 +23,7 @@
 REPO_ROOT = os.path.abspath(os.environ['REPO_ROOT'])
 PKG_ROOT = os.path.join(REPO_ROOT, 'build', 'CodeChecker')
 
+os.environ["CC_DATA_FILES_DIR"] = PKG_ROOT
+os.environ["CC_LIB_DIR"] = os.path.join(PKG_ROOT, "lib", "python3")
+
 sys.path.append(os.path.join(PKG_ROOT, 'lib', 'python3'))

web/tests/functional/diff_cmdline/__init__.py

@@ -0,0 +1,8 @@
+# coding=utf-8
+# -------------------------------------------------------------------------
+#
+#  Part of the CodeChecker project, under the Apache License v2.0 with
+#  LLVM Exceptions. See LICENSE for license information.
+#  SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+# -------------------------------------------------------------------------