use defer instead of os.Exit(m.Run())

[pmenglund]

as of go1.15 testing.M returns the return code of m.Run(), so it is possible to use defer

Related Issue or Design Document

golang/go#34129

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

n/a

[CLAassistant]

All committers have signed the CLA.

[pmenglund]

this fails on something unrelated to the PR:

Error: ../../../go/pkg/mod/github.com/docker/cli@v24.0.1+incompatible/cli/compose/template/template.go:242:14: undefined: strings.Cut

do you want me to fix the issue in this PR or a separate one?