Makefile.am: set TESTS_ENVIRONMENT to the same value as AM_TESTS_ENVIRONMENT

according to the automake documentation, the AM_TESTS_ENVIRONMENT
variable is reserved for the developer, while the TESTS_ENVIRONMENT
variable is reserved for the user.

unfortunately, setting AM_TESTS_ENVIRONMENT doesn't appear to work with
the old version of automake that travis-ci uses, so hack around it by
also setting TESTS_ENVIRONMENT.