test-lib.sh: Use GLIBC_TUNABLES instead of MALLOC_CHECK_ on glibc >= 2.34

In glibc >= 2.34 MALLOC_CHECK_ and MALLOC_PERTURB_ environment
variables have been replaced by GLIBC_TUNABLES.  Also the new
glibc requires that you preload a library called libc_malloc_debug.so
to get these features.

Using the ordinary glibc system variable detect if this is glibc >= 2.34 and
use GLIBC_TUNABLES and the new library.

This patch was inspired by a Richard W.M. Jones ndbkit patch

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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