test: split up into individual targets, run all

"make test" used to abort after the first test failure. That was
partly intentional: if the simple tests already fail (for example,
because of a syntax error), then there is no point in continuing to
test.

However, it also makes it harder to find all errors in a CI system
when the errors are unrelated (first error shows up, gets fixed, next
error shows up, etc.).

Now "make test" still aborts early, but "make -k test" is used in the
CI and will run all individual tests because they are split up into
different targets.

travis.yml

@@ -6,7 +6,7 @@ matrix:
   include:
   - go: 1.11.1
 script:
-- make all test
+- make -k all test
 after_success:
   - if [ "${TRAVIS_PULL_REQUEST}" == "false" ]; then
       docker login -u "${DOCKER_USERNAME}" -p "${DOCKER_PASSWORD}" quay.io;