Add test-short target and use it in CI

This is reinstating the old behavior. The longer tests are not only
long but also flaky. (The former is by design, the latter is a bug.)

Signed-off-by: beorn7 <beorn@soundcloud.com>

.travis.yml

@@ -8,5 +8,5 @@ go:
   - 1.10.x
 
 script:
-  - make check_license style unused test
+  - make check_license style unused test-short
   - if [[ ! $TRAVIS_GO_VERSION =~ ^1\.(7|8)\.[x0-9]+$ ]]; then make staticcheck; fi

Makefile

@@ -20,3 +20,6 @@ get_dep:
 
 .PHONY: test
 test: get_dep common-test
+
+.PHONY: test-short
+test-short: get_dep common-test-short