[merge][1.2][#189771] failure to load tests is fatal; fix broken lp plugin tests

[Martin Pool]

#189771 complains that if we fail to import the tests from a plugin, bzr
just prints a warning.  This is a problem because it means a typo or
incorrect import line for example in a plugin can cause it not to be
tested.

This patch causes the importerror to just propagate, making bzr selftest
fail.

I know from time to time people have plugins that are out of date, so
fail selftests, and want to ignore them.  This does not change
--no-plugins, and most out-of-date api usage will not trip this.

The specific case reported here is that Tim omitted to add one file in
the original patch, and the failure was hidden by this bug.  I've added
the file here, and cleaned up some incidental typos and formatting
errors.

I would like to merge this for 1.2 today.

-- 
Martin
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