Just change ENV and restore it afterwards.

Stubbing ENV[] is not safe outside MRI. At some point after the
stubbing has occurred a backtrace is printed to the ActiveSupport
warning log: there Rubinius accesses ENV['RBX_NOCOLOR'] to determine
if it should print the backtrace with colors or not, causing the
stub to fail. Other implementations might access ENV in a different
way too, we just can't predict it.
The only thing we can do here is to actually set the ENV with what
we want and restore it afterwards.
Assert presence of "frozen" in error message, not the full MRI message.

Related to all the other issues regarding message independent assertions
to make Rails compatible with other Ruby implementations other than MRI.

The best way here would be to have a specific error raised when modifying
frozen objects, like FrozenObjectError or something. But since Ruby
doesn't provide such a thing, we must limit the assertion to the lowest
common denominator, which is word "frozen".
Allow methods arity below -1 in assert_responds.

Every method from MRI's core classes is written in C. This means
Method#arity always returns -1 for methods with a variable number of
arguments. This is not the case with Rubinius, where, for example
Array#slice! is implemented in Ruby and has arity -2, since is
defined as def slice!(start, length = undefined)
Merge pull request #12456 from razielgn/ar-test-fixes

Fixed MRI dependent stuff in AR tests