Consolidating compile-fail and ui testing, and possibly others as well

[petrochenkov]

@nikomatsakis

So this is the precise change then that I propose:

Move all compile-fail tests to ui tests. Simplify to just support //~ ERROR and //~ WARNING; go back to grepping the output instead of parsing JSON. Also, just support matching the "main" message, which is easy to grep Remove all the //~ NOTE, //~ HELP, etc, because those are complicated to grep

Generate reference tests, naturally

This is exactly what I'd like to see as well. Former compile-fail tests still ensure that erroneous code produces errors, and former ui tests still ensure formatting is good + cross checks for extra reliability, and all this without having to write something like

//~^ NOTE
//~| NOTE 
//~| NOTE
//~| HELP
//~| HELP
//~| HELP

manually.