test: Use lua to perform sanity API checks

Sanity API checks made by the python-client in the api-python travis target were
converted to lua and will now live in this repository. This will simplify
performing breaking changes to the API as it won't be necessary to send parallel
PRs the python-client.

test/functional/api/server_requests_spec.lua

@@ -0,0 +1,68 @@
+-- Tests for some server->client RPC scenarios. Note that unlike with
+-- `rpcnotify`, to evaluate `rpcrequest` calls we need the client event loop to
+-- be running.
+local helpers = require('test.functional.helpers')
+local clear, nvim, eval, eq, run, stop = helpers.clear, helpers.nvim,
+      helpers.eval, helpers.eq, helpers.run, helpers.stop
+
+
+describe('server -> client', function()
+  local cid
+
+  before_each(function()
+    clear()
+    cid = nvim('get_api_info')[1]
+  end)
+
+  describe('simple call', function()
+    it('works', function()
+      local function on_setup()
+        eq({4, 5, 6}, eval('rpcrequest('..cid..', "scall", 1, 2, 3)'))
+        stop()
+      end
+
+      local function on_request(method, args)
+        eq('scall', method)
+        eq({1, 2, 3}, args)
+        nvim('command', 'let g:result = [4, 5, 6]')
+        return eval('g:result')
+      end
+      run(on_request, nil, on_setup)
+    end)
+  end)
+
+  describe('recursive call', function()
+    it('works', function()
+      local function on_setup()
+        nvim('set_var', 'result1', 0)
+        nvim('set_var', 'result2', 0)
+        nvim('set_var', 'result3', 0)
+        nvim('set_var', 'result4', 0)
+        nvim('command', 'let g:result1 = rpcrequest('..cid..', "rcall", 2)')
+        eq(4, nvim('get_var', 'result1'))
+        eq(8, nvim('get_var', 'result2'))
+        eq(16, nvim('get_var', 'result3'))
+        eq(32, nvim('get_var', 'result4'))
+        stop()
+      end
+
+      local function on_request(method, args)
+        eq('rcall', method)
+        local n = unpack(args) * 2
+        if n <= 16 then
+          local cmd
+          if n == 4 then
+            cmd = 'let g:result2 = rpcrequest('..cid..', "rcall", '..n..')'
+          elseif n == 8 then
+            cmd = 'let g:result3 = rpcrequest('..cid..', "rcall", '..n..')'
+          elseif n == 16 then
+            cmd = 'let g:result4 = rpcrequest('..cid..', "rcall", '..n..')'
+          end
+          nvim('command', cmd)
+        end
+        return n
+      end
+      run(on_request, nil, on_setup)
+    end)
+  end)
+end)