Revert "tests: remove some no longer relevant bitrot in the test environment file"

This reverts commit 107ad34f8f.

Breaks autoconf.

tests/test_env.sh.in

@@ -30,6 +30,9 @@ Msys|CYGWIN*) PATH_SEP=";";;
 Haiku) LIBRARY_PATH_ENV="BELIBRARIES";;
 esac
 
+prefix="@prefix@"
+exec_prefix="@exec_prefix@"
+datarootdir="@datarootdir@"
 pcpath="@PKG_DEFAULT_PATH@"
 
 tests_init()