Fix failure exit code regression

[jonsmock]

dctest should exit 1 if any test fails. Change the final assertion in
core.cljs to be a positive one: unless we see a :passed result, exit 1.
This should make it more difficult to introduce this kind of bug while
refactoring in the future.

We didn't catch this regression in GHA, because we were swallowing all
exit codes and only checking the results file counts.

Move integration test running into ./test/runexamples, so we can test
all examples locally more easily. Add exit code checking. Also support
running all examples with nbb as well as with a built image.

[kanaka]

Sigh, nobody has properly solved the CI runs the same locally as in the cloud problem yet. Either something like nektos/act, or some way for test scripts to output metadata so that GHA can pick up the step definitions from the test scripts (matrix strategy works at the job level, but we need something like that and more powerful at the step level too).