In the developer guide, forbid new global reactor unit tests

[exarkun]

On the mailing list, < http://twistedmatrix.com/pipermail/twisted-python/2011-November/024732.html>, proposed the idea that we shouldn't be adding new unit tests to Twisted which use the global reactor. No disagreement in the resulting discussion.

[exarkun]

(In [33092]) Branching to 'no-global-reactor-testing-5371'

[frap]

Documentation looks good and understandable. no formatiing errors nor spellling mistakes in browser. Makes sense to me.

[itamar]

When reviewing branches, if you're done with the review you should remove the "review" keyword and reassign back to author, not close the ticket.

[thijs]

Looks good to merge.

[itamar]

There's a spelling mistake: "unweildy" instead of "unwieldy". Also maybe change the title from "Tips" to "Guidelines".

[exarkun]

(In [33115]) incorporate itamar's suggested improvements

refs #5371

[exarkun]

(In [33116]) Merge no-global-reactor-testing-5371

Author: exarkun Reviewer: frap, thijs, itamar Fixes: #5371

Codify the proscription on global reactor use in unit tests.