Test schema_version_check directly

Testing via db:migrate causes side-effects like setting
ActiveRecord::Base.configurations and also it runs schema dump into
db/schema.rb. Rather than keep up, we test schema_version_check directly

spec/tasks/migrate/schema_check_rake_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 require 'rake'
 
-describe 'schema_version_check rake task', :quarantine do
+describe 'schema_version_check rake task' do
   include StubENV
 
   before :all do
@@ -15,8 +15,6 @@ describe 'schema_version_check rake task', :quarantine do
   end
 
   before do
-    # Stub out db tasks
-    allow(ActiveRecord::Tasks::DatabaseTasks).to receive(:migrate).and_return(true)
     allow(ActiveRecord::Migrator).to receive(:current_version).and_return(Gitlab::Database::MIN_SCHEMA_VERSION)
 
     # Ensure our check can re-run each time
@@ -24,23 +22,23 @@ describe 'schema_version_check rake task', :quarantine do
   end
 
   it 'allows migrations on databases meeting the min schema version requirement' do
-    expect { run_rake_task('db:migrate') }.not_to raise_error
+    expect { run_rake_task('schema_version_check') }.not_to raise_error
   end
 
   it 'raises an error when schema version is too old to migrate' do
     allow(ActiveRecord::Migrator).to receive(:current_version).and_return(25)
-    expect { run_rake_task('db:migrate') }.to raise_error(RuntimeError, /current database version is too old to be migrated/)
+    expect { run_rake_task('schema_version_check') }.to raise_error(RuntimeError, /current database version is too old to be migrated/)
   end
 
   it 'skips running validation when passed the skip env variable' do
     stub_env('SKIP_SCHEMA_VERSION_CHECK', 'true')
     allow(ActiveRecord::Migrator).to receive(:current_version).and_return(25)
-    expect { run_rake_task('db:migrate') }.not_to raise_error
+    expect { run_rake_task('schema_version_check') }.not_to raise_error
   end
 
   it 'allows migrations on fresh databases' do
     allow(ActiveRecord::Migrator).to receive(:current_version).and_return(0)
-    expect { run_rake_task('db:migrate') }.not_to raise_error
+    expect { run_rake_task('schema_version_check') }.not_to raise_error
   end
 
   def run_rake_task(task_name)