[dotest] Remove long running test "decorator" and re-enable tests.

Today I discovered the skipLongRunningTest decorator and to my surprise
all the tests were passing without the decorator. They don't seem to be
that expensive either, they take a few seconds but we have tests that
take much longer than that. As such I propose to remove the decorator
and enable them by default.

Differential revision: https://reviews.llvm.org/D66774

llvm-svn: 369995