Stop running tidy on the test suite

[marijnh]

Having the build fail because a test includes a long line seems a bad use of everybody's time.

[nikomatsakis]

I am inclined to agree, though the last time I proposed it @brson was
opposed. I did make it so that lines which include "//!" are exempted,
allowing you to include long expected error messages.

[brson]

I am in favor now. Probably should convince graydon too. Lets remember to remove the special case from the tidy script.

[catamorphism]

I'm in favor.

[brson]

@graydon do you have an opinion?

[brson]

Done.