Serial execution on iOS devices is not specific to sanitizers. We want
to throttle all on-device tests. Pull the setting of the
parallelism_group up into the common lit configuration file.
Rename darwin-ios-device-sanitizer to ios-device. This group is not
specific to sanitizers and (theoretically) independent from the host OS.
Note that we don't support running unit tests on-device (there are no
configurations generated for that). If that ever changes, we also need
this configuration in unittests/lit.common.unit.cfg.