Rename the erlang test_server module

erlang/otp began shipping with an app called test_server since November 2009 with R13B03. The current version of erlang/otp is approaching R18.

Thrift's test also comes with test_server.erl at lib/erl/test/test_server.erl as well as a test_client.erl

after compiling Thrift's erlang modules, I was expecting to find test_server:go/0 but instead found functions from https://github.com/erlang/otp/blob/master/lib/test_server/src/test_server.erl

To avoid such conflicts while developing and in erlang deployments/releases, test_server.erl can be renamed as test_thrift_server.erl.