Changes to test/example.c:

• Passing --force_fail injects a test failure (for use when debugging how CI handles junit)
• Passing --fail_fast causes first failure to abort (now that Chris's change makes continuing easy)
• Passing --junit foo.xml no longer disables normal output (otherwise it felt like flying blind)
• --junit's output is now compatible with cirrus-ci's junit parser
  • When cirrus-ci builds fail, Github displays test failures with links to source lines
    (see example at https://github.com/dankegel/zlib/pull/3/checks?check_run_id=690339991)
  • If the failed build is in a pull request, the failures also show up in the 'Files changed' tab (see example at https://github.com/dankegel/zlib/pull/3/files and scroll down to the 'Unchanged files with check annotations' section)
• Options can now be set via the environment, e.g. setting ZLIBTEST_FORCE_FAIL=1 is equivalent to passing --force_fail

Other changes:

• appveyor now runs tests via ctest
• Added .cirrus.yml to enable cirrus-ci builds
• README now shows status badges for both Appveyor and Cirrus; see https://github.com/dankegel/zlib/tree/develop for what it looks like when the git repo has those two services enabled