Only use the test_io manager if the right environment variables are set

Only use the test_io manager if the right environment variables are set

In order to make it possible for the test_io manager to be compiled in
by default, make all of the programs that might try to use it to only
do so if the environment variables TEST_IO_FLAGS and TEST_IO_DEBUG are
set.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>