tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues fou

tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues found along the way.

Cherry-picked from #13489
Closes #13600

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tests: propagate errors in libtests

Use the test macros to automatically propagate some errors, and check
and log others while running the tests. This can help in debugging
exactly w

tests: propagate errors in libtests

Use the test macros to automatically propagate some errors, and check
and log others while running the tests. This can help in debugging
exactly why a test has failed.

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copyright: update all copyright lines and remove year ranges

- they are mostly pointless in all major jurisdictions
- many big corporations and projects already don't use them
- save

copyright: update all copyright lines and remove year ranges

- they are mostly pointless in all major jurisdictions
- many big corporations and projects already don't use them
- saves us from pointless churn
- git keeps history for us
- the year range is kept in COPYING

checksrc is updated to allow non-year using copyright statements

Closes #10205

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test1948: verify PUT + POST reusing the same handle

Reproduced #9507, verifies the fix