Tests: Disable 'stdbuf' when TAP autotime is disabled

Tests: Disable 'stdbuf' when TAP autotime is disabled

The 'stdbuf' command is used by default in 'tap-driver.sh' to force line
buffering. It was added to help with TAP autotime output to log files.

However, 'stdbuf' causes issues in our 32-64 integration tests where we
mix 32 and 64 bit binaries. It uses an LD_PRELOAD library that is not
in a multiarch path which results in the following warning message on
stderr when a 32-bit binary is executed on a 64-bit system:

  ERROR: ld.so: object '/usr/libexec/coreutils/libstdbuf.so' from
  LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS64): ignored.

Many of our tests compare the content of stderr to an expected file
which results in their failure.

We already have an environment variable "TAP_AUTOTIME" to disable the
autotime feature, make it disable the use of 'stdbuf' as well.

Change-Id: I307cbfcddd7772f69e8211c51b03fb9a3da8e841
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>