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/tabpage_spec.lua

@@ -0,0 +1,42 @@
+-- Sanity checks for tabpage_* API calls via msgpack-rpc
+local helpers = require('test.functional.helpers')
+local clear, nvim, tabpage, curtab, eq, ok =
+  helpers.clear, helpers.nvim, helpers.tabpage, helpers.curtab, helpers.eq,
+  helpers.ok
+
+describe('tabpage_* functions', function()
+  before_each(clear)
+
+  describe('get_windows and get_window', function()
+    it('works', function()
+      nvim('command', 'tabnew')
+      nvim('command', 'vsplit')
+      local tab1, tab2 = unpack(nvim('get_tabpages'))
+      local win1, win2, win3 = unpack(nvim('get_windows'))
+      eq({win1},  tabpage('get_windows', tab1))
+      eq({win2, win3},  tabpage('get_windows', tab2))
+      eq(win2, tabpage('get_window', tab2))
+      nvim('set_current_window', win3)
+      eq(win3, tabpage('get_window', tab2))
+    end)
+  end)
+
+  describe('{get,set}_var', function()
+    it('works', function()
+      curtab('set_var', 'lua', {1, 2, {['3'] = 1}})
+      eq({1, 2, {['3'] = 1}}, curtab('get_var', 'lua'))
+      eq({1, 2, {['3'] = 1}}, nvim('eval', 't:lua'))
+    end)
+  end)
+
+  describe('is_valid', function()
+    it('works', function()
+      nvim('command', 'tabnew')
+      local tab = nvim('get_tabpages')[2]
+      nvim('set_current_tabpage', tab)
+      ok(tabpage('is_valid', tab))
+      nvim('command', 'tabclose')
+      ok(not tabpage('is_valid', tab))
+    end)
+  end)
+end)