rustbuild: Don't run pretty tests by default

This commit relegates all pretty tests to not get run by default and rather get
run as part of an "aux" test suite. This "aux" suite is renamed from the old
"cargotest" suite to just collect tests that don't need to run everywhere but
should at least pass on Unix/Windows.

.travis.yml

@@ -19,7 +19,7 @@ matrix:
     - env: IMAGE=i686-gnu-nopt
     - env: IMAGE=x86_64-freebsd
     - env: IMAGE=x86_64-gnu
-    - env: IMAGE=x86_64-gnu-cargotest
+    - env: IMAGE=x86_64-gnu-aux
     - env: IMAGE=x86_64-gnu-debug
     - env: IMAGE=x86_64-gnu-nopt
     - env: IMAGE=x86_64-gnu-make