Make the `test/run` script compatible to the ancient Bash version on macos.
```
test/run: line 56: conditional binary operator expected
```
Also, run `set -e` only for Bash >= 4. Otherwise the shell silently exits with
a failure when a command (expectedly) fails.
fix bogus test failure with Bash 3.2 on macos
Use Github actions for CI
Integration with Travis CI (org) is broken for a few months already, and travis-ci.org is going to be shut down in favor of travis-ci.com which does not allow builds on macos freely.
