[lldb/test] Change base class of lldb-server tests

lldb-server tests are a very special subclass of "api" tests. As they
communicate with lldb-server directly, they don't actually need most of
facilities provided by our TestBase class. In particular, they don't
need the ability to fork debug info flavours of tests (but they could
use debug server flavours).

This makes them inherit from "Base" instead. This avoids the need to
explicitly mark these tests as NO_DEBUG_INFO_TEST_CASE. Two additional
necessary tweaks were:
- move run_platform_command to the base (Base) class. This is used in
  one test, and can be generally useful when running tests remotely.
- add a "build" method, forwarding to buildDefault. This is to avoid
  updating each test case to use buildDefault (also, "build" sounds
  better). It might be interesting to refactor the (Test)Base classes so
  that all debug info flavour handling happens in TestBase, and the Base
  class provides a simple build method automatically.