Default mix test compiles a single locale, making cold test builds faster

A fresh mix test on a new checkout no longer regenerates the ex_cldr backend for all ten locales just to run tests. The Test environment now compiles only the English locale by default (matching the existing Dev behavior), and the i18n-asserting tests are tagged :locale so they are excluded unless TUIST_DEV_ALL_LOCALES=1 is set (CI sets it, so coverage there is unchanged). Result: the cold _build/test compile on a fresh worktree is faster, while production behavior and CI coverage stay the same.