diff --git a/core/modules/simpletest/tests/bootstrap.test b/core/modules/simpletest/tests/bootstrap.test
index 88f3d41feca..fee8fcc0dd1 100644
--- a/core/modules/simpletest/tests/bootstrap.test
+++ b/core/modules/simpletest/tests/bootstrap.test
@@ -116,7 +116,9 @@ class BootstrapPageCacheTestCase extends DrupalWebTestCase {
    * Test support for requests containing If-Modified-Since and If-None-Match headers.
    */
   function testConditionalRequests() {
-    variable_set('cache', 1);
+    $config = config('system.performance');
+    $config->set('cache', 1);
+    $config->save();
 
     // Fill the cache.
     $this->drupalGet('');
@@ -154,7 +156,9 @@ class BootstrapPageCacheTestCase extends DrupalWebTestCase {
    * Test cache headers.
    */
   function testPageCache() {
-    variable_set('cache', 1);
+    $config = config('system.performance');
+    $config->set('cache', 1);
+    $config->save();
 
     // Fill the cache.
     $this->drupalGet('system-test/set-header', array('query' => array('name' => 'Foo', 'value' => 'bar')));
@@ -198,7 +202,9 @@ class BootstrapPageCacheTestCase extends DrupalWebTestCase {
    * mod_deflate Apache module.
    */
   function testPageCompression() {
-    variable_set('cache', 1);
+    $config = config('system.performance');
+    $config->set('cache', 1);
+    $config->save();
 
     // Fill the cache and verify that output is compressed.
     $this->drupalGet('', array(), array('Accept-Encoding: gzip,deflate'));
diff --git a/core/modules/simpletest/tests/session.test b/core/modules/simpletest/tests/session.test
index 846f6d314c7..cca23bad6ea 100644
--- a/core/modules/simpletest/tests/session.test
+++ b/core/modules/simpletest/tests/session.test
@@ -139,7 +139,10 @@ class SessionTestCase extends DrupalWebTestCase {
     $this->assertSessionEmpty(TRUE);
 
     // The same behavior is expected when caching is enabled.
-    variable_set('cache', 1);
+    $config = config('system.performance');
+    $config->set('cache', 1);
+    $config->save();
+
     $this->drupalGet('');
     $this->assertSessionCookie(FALSE);
     $this->assertSessionEmpty(TRUE);
@@ -455,7 +458,7 @@ class SessionHttpsTestCase extends DrupalWebTestCase {
       }
     }
 
-    // Test that session data saved before login is not available using the 
+    // Test that session data saved before login is not available using the
     // pre-login anonymous cookie.
     $this->cookies = array();
     $this->drupalGet('session-test/get', array('Cookie: ' . $anonymous_cookie));
diff --git a/core/modules/statistics/statistics.test b/core/modules/statistics/statistics.test
index 592f1653c94..7fac22c3535 100644
--- a/core/modules/statistics/statistics.test
+++ b/core/modules/statistics/statistics.test
@@ -69,7 +69,9 @@ class StatisticsLoggingTestCase extends DrupalWebTestCase {
     $this->node = $this->drupalCreateNode(array('title' => $this->randomName(255), 'uid' => $this->auth_user->uid));
 
     // Enable page caching.
-    variable_set('cache', TRUE);
+    $config = config('system.performance');
+    $config->set('cache', 1);
+    $config->save();
 
     // Enable access logging.
     variable_set('statistics_enable_access_log', 1);