server_requests_spec: fix assertion, pass Lua paths via args (#10875)

This makes it pick up the nvim Luarocks module properly when not
installed via third-party.

test/functional/api/rpc_fixture.lua

@@ -1,12 +1,8 @@
-local deps_prefix = (os.getenv('DEPS_PREFIX') and os.getenv('DEPS_PREFIX')
-                     or './.deps/usr')
-
-package.path = deps_prefix .. '/share/lua/5.1/?.lua;' ..
-               deps_prefix .. '/share/lua/5.1/?/init.lua;' ..
-               package.path
-
-package.cpath = deps_prefix .. '/lib/lua/5.1/?.so;' ..
-                package.cpath
+--- RPC server fixture.
+--
+-- Lua's paths are passed as arguments to reflect the path in the test itself.
+package.path = arg[1]
+package.cpath = arg[2]
 
 local mpack = require('mpack')
 local StdioStream = require('nvim.stdio_stream')