Travis CI: Only run fast testsuites

We could run all testsuites, each in dedicated jobs, because separately,
they should take less than 50 minutes. However, this would bloat the
build page and blocks other jobs for a significant time. Using `ct-fast`
should be enough in Travis CI. All testsuites are still running in
Concourse anyway.

[#152509619]

Author: dumbbell

.travis.yml

@@ -37,7 +37,7 @@ before_script:
 
 script:
   - make xref
-  - make tests
+  - make ct-fast
 
 after_failure:
   - |

.travis.yml.patch

@@ -0,0 +1,11 @@
+--- .travis.yml.orig	2017-11-08 14:23:22.036394000 +0100
++++ .travis.yml	2017-11-08 15:26:19.264520000 +0100
+@@ -37,7 +37,7 @@
+ 
+ script:
+   - make xref
+-  - make tests
++  - make ct-fast
+ 
+ after_failure:
+   - |