Don't attempt to specify datetime precision unless supported

Specifically, versions of MySQL prior to 5.6 do not support this, which
is what's used on Travis by default. The method `mysql_56?` appeared to
only ever be used to conditionally apply subsecond precision, so I've
generalized it and used it more liberally.

This should fix the test failures caused by #20317

activerecord/test/cases/base_test.rb

@@ -204,7 +204,7 @@ class BasicsTest < ActiveRecord::TestCase
     )
 
     # For adapters which support microsecond resolution.
-    if current_adapter?(:PostgreSQLAdapter, :SQLite3Adapter) || mysql_56?
+    if subsecond_precision_supported?
       assert_equal 11, Topic.find(1).written_on.sec
       assert_equal 223300, Topic.find(1).written_on.usec
       assert_equal 9900, Topic.find(2).written_on.usec

activerecord/test/cases/helper.rb

@@ -46,10 +46,10 @@ def in_memory_db?
   ActiveRecord::Base.connection_pool.spec.config[:database] == ":memory:"
 end
 
-def mysql_56?
-  current_adapter?(:MysqlAdapter, :Mysql2Adapter) &&
-    ActiveRecord::Base.connection.send(:version) >= '5.6.0' &&
-    ActiveRecord::Base.connection.send(:version) < '5.7.0'
+def subsecond_precision_supported?
+  !current_adapter?(:MysqlAdapter, :Mysql2Adapter) ||
+    (ActiveRecord::Base.connection.send(:version) >= '5.6.0' &&
+    ActiveRecord::Base.connection.send(:version) < '5.7.0')
 end
 
 def mysql_enforcing_gtid_consistency?

activerecord/test/schema/schema.rb

@@ -251,10 +251,17 @@ ActiveRecord::Schema.define do
   create_table :developers, force: true do |t|
     t.string   :name
     t.integer  :salary, default: 70000
+    if subsecond_precision_supported?
       t.datetime :created_at, precision: 6
       t.datetime :updated_at, precision: 6
       t.datetime :created_on, precision: 6
       t.datetime :updated_on, precision: 6
+    else
+      t.datetime :created_at
+      t.datetime :updated_at
+      t.datetime :created_on
+      t.datetime :updated_on
+    end
   end
 
   create_table :developers_projects, force: true, id: false do |t|
@@ -353,7 +360,11 @@ ActiveRecord::Schema.define do
 
   create_table :invoices, force: true do |t|
     t.integer :balance
+    if subsecond_precision_supported?
       t.datetime :updated_at, precision: 6
+    else
+      t.datetime :updated_at
+    end
   end
 
   create_table :iris, force: true do |t|
@@ -503,7 +514,11 @@ ActiveRecord::Schema.define do
 
   create_table :owners, primary_key: :owner_id, force: true do |t|
     t.string :name
+    if subsecond_precision_supported?
       t.column :updated_at, :datetime, precision: 6
+    else
+      t.column :updated_at, :datetime
+    end
     t.column :happy_at,   :datetime
     t.string :essay_id
   end
@@ -755,7 +770,7 @@ ActiveRecord::Schema.define do
     t.string   :title, limit: 250
     t.string   :author_name
     t.string   :author_email_address
-    if mysql_56?
+    if subsecond_precision_supported?
       t.datetime :written_on, precision: 6
     else
       t.datetime :written_on
@@ -778,7 +793,11 @@ ActiveRecord::Schema.define do
     t.string   :parent_title
     t.string   :type
     t.string   :group
+    if subsecond_precision_supported?
       t.timestamps null: true, precision: 6
+    else
+      t.timestamps null: true
+    end
   end
 
   create_table :toys, primary_key: :toy_id, force: true do |t|