test: refer to storage server by id

Index alias provide stable info on instance nature, yet servers list
index doesn't.

Part of #412
Part of #415

Author: DifferentialOrange

test/integration/borders_test.lua

@@ -12,7 +12,7 @@ local pgroup = t.group('borders', helpers.backend_matrix({
 pgroup.before_all(function(g)
     helpers.start_default_cluster(g, 'srv_select')
 
-    g.space_format = g.cluster.servers[2].net_box.space.customers:format()
+    g.space_format = g.cluster:server('s1-master').net_box.space.customers:format()
 end)
 
 pgroup.after_all(function(g)

test/integration/custom_bucket_id_test.lua

@@ -12,7 +12,7 @@ local pgroup = t.group('custom_bucket_id', helpers.backend_matrix({
 pgroup.before_all(function(g)
     helpers.start_default_cluster(g, 'srv_simple_operations')
 
-    g.space_format = g.cluster.servers[2].net_box.space.customers:format()
+    g.space_format = g.cluster:server('s1-master').net_box.space.customers:format()
 end)
 
 pgroup.after_all(function(g)

test/integration/pairs_test.lua

@@ -13,7 +13,7 @@ local pgroup = t.group('pairs', helpers.backend_matrix({
 pgroup.before_all(function(g)
     helpers.start_default_cluster(g, 'srv_select')
 
-    g.space_format = g.cluster.servers[2].net_box.space.customers:format()
+    g.space_format = g.cluster:server('s1-master').net_box.space.customers:format()
 
     g.cluster.main_server.net_box:eval([[
         require('crud').cfg{ stats = true }

test/integration/select_readview_test.lua

@@ -16,7 +16,7 @@ local pgroup = t.group('select_readview', helpers.backend_matrix({
 local function init_cluster(g)
     helpers.start_default_cluster(g, 'srv_select')
 
-    g.space_format = g.cluster.servers[2].net_box.space.customers:format()
+    g.space_format = g.cluster:server('s1-master').net_box.space.customers:format()
 
     g.router = helpers.get_router(g.cluster, g.params.backend)
     g.router.net_box:eval([[

test/integration/select_test.lua

@@ -14,7 +14,7 @@ local pgroup = t.group('select', helpers.backend_matrix({
 pgroup.before_all(function(g)
     helpers.start_default_cluster(g, 'srv_select')
 
-    g.space_format = g.cluster.servers[2].net_box.space.customers:format()
+    g.space_format = g.cluster:server('s1-master').net_box.space.customers:format()
 
     g.router = helpers.get_router(g.cluster, g.params.backend)
     g.router.net_box:eval([[

test/integration/truncate_test.lua

@@ -10,7 +10,7 @@ local pgroup = t.group('truncate', helpers.backend_matrix({
 pgroup.before_all(function(g)
     helpers.start_default_cluster(g, 'srv_select')
 
-    g.space_format = g.cluster.servers[2].net_box.space.customers:format()
+    g.space_format = g.cluster:server('s1-master').net_box.space.customers:format()
 end)
 
 pgroup.after_all(function(g)