Tests: Switch to fedora-25 in test plugin

Use fedora-25 Vagrant box in VagrantTestPlugin, which was missing from
9a3ab3e8000be059ccf4edeee53b62ddb5913fc6 causing packaging test
failures.

Additionally update TESTING.asciidoc

TESTING.asciidoc

@@ -350,7 +350,7 @@ These are the linux flavors the Vagrantfile currently supports:
 * debian-8 aka jessie, the current debian stable distribution
 * centos-6
 * centos-7
-* fedora-24
+* fedora-25
 * oel-6 aka Oracle Enterprise Linux 6
 * oel-7 aka Oracle Enterprise Linux 7
 * sles-12
@@ -436,13 +436,13 @@ vagrantUbuntu1604#up.
 Once up, you can then connect to the VM using SSH from the elasticsearch directory:
 
 -------------------------------------------------
-vagrant ssh fedora-24
+vagrant ssh fedora-25
 -------------------------------------------------
 
 Or from another directory:
 
 -------------------------------------------------
-VAGRANT_CWD=/path/to/elasticsearch vagrant ssh fedora-24
+VAGRANT_CWD=/path/to/elasticsearch vagrant ssh fedora-25
 -------------------------------------------------
 
 Note: Starting vagrant VM outside of the elasticsearch folder requires to

buildSrc/src/main/groovy/org/elasticsearch/gradle/vagrant/VagrantTestPlugin.groovy

@@ -17,7 +17,7 @@ class VagrantTestPlugin implements Plugin<Project> {
             'centos-6',
             'centos-7',
             'debian-8',
-            'fedora-24',
+            'fedora-25',
             'oel-6',
             'oel-7',
             'opensuse-13',