Replace Boost.Test with lightweight_test from Boost.Core

[mloskot]

The tests do not require fully-featured testing framework.
The lightweight_test alternative should also save on build times.

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Motivation is drawn from recent discussions on #boost channel at cpplang.slack.com, about preferring the lightweight_test wherever possible, also to cut number of inter-dependencies in Boost.

[mclow]

@mloskot - the problem with discussions on the slack channel (as opposed to the mailing list) is that they are not archived, and can't be reviewed later.

[mloskot]

@mclow Right. However, I think that even if I did not mention it, the rationale behind the change should be clear enough.

[pdimov]

You forgot to return boost::report_errors(); :-)

[mclow]

It's not the motivation that I'm missing, but rather the discussion of the pros and the cons.

[mloskot]

@pdimov Thank you. I've updated the pull request.

@mclow The discussion was not very extensive, just that for simple needs lightweight_test should be enough, less dependency may decrease overall maintenance hassle, and such.

Quoting @mjcaisse

Boost.Test is in a peculiar position of being a dependency for so many pre-c++11 supporting libraries. As such it has a different set of constraints.

[mclow]

Committed a modified version of this as 453cf59