run-tests: set a default largefiles usercache in the default hgrc file This fixes a test failure introduced in 4be754832829 on Windows and OS X, where the cached largefile wasn't being deleted because the named .cache directory didn't exist. It only existed on Linux because the test suite sets $HOME to the directory of the test being run, and Linux uses $HOME/.cache by default. Most of the other largefiles tests explicitly set this value at the top of their scripts, but test-largefiles-update.t didn't pick that up when it was created. Those scripts that do set a value will override this. We could just set the parameter in the test-largefiles-update.t script, but there are a few other non obvious tests that exercise largefiles too. These largefiles end up being cached in the user's real cache, so proper hygiene dictates that this not be left to each individual test script.