gdb, testsuite: Rename set_sanitizer procedures to append_environment.

The procedures set_sanitizer_1, set_sanitizer and set_sanitizer_default
are used for the configuration of ASAN specific environment variables.
However, they are actually generic.  Rename them to append_environment*
so that their purpose is more clear.

Approved-By: Tom Tromey <tom@tromey.com>
6 files changed