Use the env TEMPDIR if available instead of P_tmpdir.

[ironage]

On Mac this changes the directory that test files are written from "/var/tmp/" to "/var/folders/1y/_7..." which allows the OS to clean them up on a reboot.

Additionally, TestFile will clean up the temp directory it creates when finished, if not instructed to persist it.

This should mitigate part of https://jira.mongodb.org/browse/BUILD-13876 going forward.

My dev machine had 2 million mostly empty directories in /var/tmp/realm_* totalling 12 GB. Every run of the object-store tests created around 4000 directories there. Since at this scale, most attempts to do anything at that path from the terminal didn't work I had to use a script to clean it out:

            util::DirScanner dir("/var/tmp/");
            size_t count = 0;
            size_t did_delete = 0;
            std::string name;
            while (dir.next(name)) {
                if (name.size() > 6 && name.substr(0, 6) == "realm_") {
                    bool deleted = util::try_remove_dir_recursive("/var/tmp/" + name);
                    did_delete = did_delete + size_t(deleted);
                }
                ++count;
                if (count % 10000 == 0) {
                    std::cout << util::format("deleted %1 out of %2", did_delete, count) << std::endl;
                }
            }
            std::cout << "end of file listings" << std::endl;

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)