Use LOG_COMPILER instead of TESTS_ENVIRONMENT in tests/Makefile.am

TESTS_ENVIRONMENT is no longer a safe place to put an Emacs
invocation.  However, LOG_COMPILER was introduced in Automake 1.12 for
such things.

Tested with Automake 1.13.1.
jabber-ping.el: require jabber-disco

Needed for jabber-advertised-features.