tests(swift): debug info

[reez]

Description

Getting intermittent failures like this on live tests on CI.

Can't reproduce locally, so trying to get more output to debug with.

The fflush function should force the standard output buffer to be written, so our diagnostic messages are more likely to appear in the CI logs even if the test subsequently crashes or encounters an assertion failure.

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[thunderbiscuit]

ACK 0a756b6.