template: proper test group usage

Test group entity is passed to each hook/test. Legacy approach is to use
group as global variable, which should work for simple groups, but is
invalid for parametrized ones. `cg` variable name was borrowed from
luatest README (it is likely a "current group" abbreviation).

cli/create/templates/cartridge/test/integration/api_test.lua

@@ -2,30 +2,29 @@ local t = require('luatest')
 local g = t.group('integration_api')
 
 local helper = require('test.helper')
-local cluster = helper.cluster
 
-g.before_all(function()
-    g.cluster = helper.cluster
-    g.cluster:start()
+g.before_all(function(cg)
+    cg.cluster = helper.cluster
+    cg.cluster:start()
 end)
 
-g.after_all(function()
-    helper.stop_cluster(g.cluster)
+g.after_all(function(cg)
+    helper.stop_cluster(cg.cluster)
 end)
 
-g.before_each(function()
+g.before_each(function(cg)
     -- helper.truncate_space_on_cluster(g.cluster, 'Set your space name here')
 end)
 
-g.test_sample = function()
-    local server = cluster.main_server
+g.test_sample = function(cg)
+    local server = cg.cluster.main_server
     local response = server:http_request('post', '/admin/api', {json = {query = '{ cluster { self { alias } } }'}})
     t.assert_equals(response.json, {data = { cluster = { self = { alias = 'api' } } }})
     t.assert_equals(server.net_box:eval('return box.cfg.memtx_dir'), server.workdir)
 end
 
-g.test_metrics = function()
-    local server = cluster.main_server
+g.test_metrics = function(cg)
+    local server = cg.cluster.main_server
     local response = server:http_request('get', '/metrics')
     t.assert_equals(response.status, 200)
     t.assert_equals(response.reason, "Ok")

cli/create/templates/cartridge/test/unit/sample_test.lua

@@ -2,11 +2,11 @@ local t = require('luatest')
 local g = t.group('unit_sample')
 
 -- create your space here
-g.before_all(function() end)
+g.before_all(function(cg) end)
 
 -- drop your space here
-g.after_all(function() end)
+g.after_all(function(cg) end)
 
-g.test_sample = function()
+g.test_sample = function(cg)
     t.assert_equals(type(box.cfg), 'table')
 end