Const suggestion unit tests write to fixed temporary directories #253

Follow-up of #217 and #239 (final review, M7).

Three unit tests of the const suggestion check in test/unit/analyzer_module_unit_tests.cpp write their inputs to a fixed directory under the system temporary directory:

• ct_const_suggestion_top_level_lost
• ct_const_suggestion_nested_identities
• ct_const_suggestion_implicit_members

Two runs of the unit tests at the same time on one machine (two build trees, or a CI runner shared by jobs) write and remove the same files, and one run can fail or read the other's inputs.

Fix: name each directory with the process id, as testConstSuggestionRewriteFailures does, or create it with a unique name.