Rewrite the "show environment" test

In a review early in the year:

    https://inbox.sourceware.org/gdb-patches/874iz3f4ek.fsf@redhat.com/

Andrew pointed out that a new test I proposed failed with read1.
This test was essentially a copy of gdb.base/environ.exp.

I couldn't reproduce the read1 problem, but this patch rewrites the
one test there that seems like it could possibly fail in this mode.
Now it uses line-by-line mode and checks for a certain environment
variable appearing in the output.

Approved-By: Simon Marchi <simon.marchi@efficios.com>