introduce new "flaky" test group

this group is skipped by default

_test/phpunit.xml

@@ -18,6 +18,12 @@
         </testsuite>
     </testsuites>
 
+    <groups>
+        <exclude>
+            <group>flaky</group>
+        </exclude>
+    </groups>
+
     <filter>
         <whitelist addUncoveredFilesFromWhitelist="false">
             <directory suffix=".php">../</directory>

_test/tests/inc/cache_use.test.php

@@ -4,6 +4,8 @@
  * Class cache_use_test
  *
  * Tests if caching can actually be used
+ *
+ * @todo tests marked as flaky until Ticket #694 has been fixed
  */
 class cache_use_test extends DokuWikiTest {
     /** @var cache_renderer $cache */
@@ -28,18 +30,11 @@ class cache_use_test extends DokuWikiTest {
         touch($this->cache->cache, $time);
     }
 
-    function test_use() {
-        $this->markTestSkipped('Disabled until Ticket #694 has been fixed');
-        return;
-
-        $this->assertTrue($this->cache->useCache());
-    }
-
     /**
      * In all the following tests the cache should not be usable
      * as such, they are meaningless if test_use didn't pass.
      *
-     * @depends test_use
+     * @group flaky
      */
     function test_purge() {
         /* @var Input $INPUT */
@@ -51,7 +46,7 @@ class cache_use_test extends DokuWikiTest {
     }
 
     /**
-     * @depends test_use
+     * @group flaky
      */
     function test_filedependency() {
         // give the dependent src file the same mtime as the cache
@@ -60,7 +55,7 @@ class cache_use_test extends DokuWikiTest {
     }
 
     /**
-     * @depends test_use
+     * @group flaky
      */
     function test_age() {
         // need to age both our source file & the cache
@@ -74,7 +69,7 @@ class cache_use_test extends DokuWikiTest {
     }
 
     /**
-     * @depends test_use
+     * @group flaky
      */
     function test_confnocaching() {
         global $conf;
@@ -83,4 +78,4 @@ class cache_use_test extends DokuWikiTest {
         $this->assertFalse($this->cache->useCache());
         $this->assertNotEmpty($this->cache->_nocache);
     }
-}
\ No newline at end of file
+}

_test/tests/inc/httpclient_http.test.php

@@ -289,6 +289,7 @@ class httpclient_http_test extends DokuWikiTest {
      * This address caused trouble with stream_select()
      *
      * @group internet
+     * @group flaky
      */
     function test_wikimatrix(){
         $http = new HTTPMockClient();