Don't override ENV['TMPDIR'] in ActionView's abstract_unit

Any attempt to override ENV['TMPDIR'] will cause problems on JRuby. like
when we call `Dir.mktmpdir` ActionView's TemplateDigestorTests. This
causes all tests to fail on JRuby.

Before Change:
JRuby: All 22 tests failing
MRI: All green

AFter this change:
JRuby: All TemplateDigestorTests green.
MRI: All green

Credits: Alex Tambellini(@ atambo)