tests: clean up tests of env -- handling

tests: clean up tests of env -- handling

The comment in env.c about -- handling has not matched the behavior
in the code since the initial commit back in 1992.

* src/env.c: Fix bogus comment.
* tests/misc/env: Further tweaks, avoiding PATH problems inherent
in testing -i, and testing program name containing =.
* doc/coreutils.texi (env invocation): Mention that intermediate
program is needed to invoke program with name containing =.