Testcases: Use Xephyr instead of XDummy

Replace the XDummy script with Xephyr. This is done because of some
changes in the Xorg server that make XDummy difficult to use.

Rename library internal variables and function names to replace "xdummy"
with "xserver" to show this change (except for renaming the package and
lib file for better git history).

Rename the switch `--keep-xdummy-output` to `--keep-xserver-output`.
This switch should now be rarely used because Xephyr requires less set
up.

Replace "xdummy" with "xephyr" in comments and utility help
information. Update docs to show the new dependency.

fixes i3#1367

Author: Tony Crisci

docs/testsuite

@@ -74,6 +74,9 @@ client, simply called +cpan+. It comes with every Perl installation and can be
 used to install the testsuite. Many users prefer to use the more modern
 +cpanminus+ instead, though (because it asks no questions and just works):
 
+The tests additionally require +Xephyr(1)+ to run a nested X server. Install
++xserver-xephyr+ on Debian or +xorg-xserver-xephyr+ on Arch Linux.
+
 .Installing testsuite dependencies using cpanminus (preferred)
 --------------------------------------------------------------------------------
 $ cd ~/i3/testcases
@@ -102,7 +105,7 @@ more testcases. Also, it takes care of starting up a separate instance of i3
 with an appropriate configuration file and creates a folder for each run
 containing the appropriate i3 logfile for each testcase. The latest folder can
 always be found under the symlink +latest/+. Unless told differently, it will
-run the tests on a separate X server instance (using the Xdummy script).
+run the tests on a separate X server instance (using Xephyr).
 
 .Example invocation of complete-run.pl+
 ---------------------------------------
@@ -146,12 +149,11 @@ $ less latest/i3-log-for-04-floating.t
 If your attempt to run the tests with a bare call to ./complete-run.pl fails, try this:
 
 ---------------------------------------------------
-$ ./complete-run.pl --parallel=1 --keep-xdummy-output
+$ ./complete-run.pl --parallel=1 --keep-xserver-output
 ---------------------------------------------------
 
-One common cause of failures is not having the X dummy server module
-installed.  Under Debian and Ubuntu this is the package
-+xserver-xorg-video-dummy+.
+This will show the output of Xephyr, which is the X server implementation we
+use for testing.
 
 ==== IPC interface
 
@@ -175,10 +177,9 @@ manager.
 === Filesystem structure
 
 In the git root of i3, the testcases live in the folder +testcases+. This
-folder contains the +complete-run.pl+ and +Xdummy+ scripts and a base
-configuration file which will be used for the tests. The different testcases
-(their file extension is .t, not .pl) themselves can be found in the
-conventionally named subfolder +t+:
+folder contains the +complete-run.pl+ and a base configuration file which will
+be used for the tests. The different testcases (their file extension is .t, not
+.pl) themselves can be found in the conventionally named subfolder +t+:
 
 .Filesystem structure
 --------------------------------------------
@@ -197,7 +198,6 @@ conventionally named subfolder +t+:
 │   │   ├── omitted for brevity
 │   │   ├── ...
 │   │   └── 74-regress-focus-toggle.t
-│   └── Xdummy
 --------------------------------------------
 
 == Anatomy of a testcase

testcases/.gitignore

@@ -8,4 +8,3 @@ inc
 META.yml
 i3-cfg-for-*
 -
-Xdummy.so