[lldb/test] Reduce boilerplate in lldb-server tests

Nearly all of our lldb-server tests have two flavours (lldb-server and
debugserver). Each of them is tagged with an appropriate decorator, and
each of them starts with a call to a matching "init" method. The init
calls are mandatory, and it's not possible to meaningfully combine them
with a different decorator.

This patch leverages the existing decorators to also tag the tests with
the appropriate debug server tag, similar to how we do with debug info
flavours. This allows us to make the "init" calls from inside the common
setUp method.