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variables have been replaced by GLIBC_TUNABLES.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>

t/test-lib.sh

@@ -518,9 +518,25 @@ else
 	setup_malloc_check () {
 		MALLOC_CHECK_=3	MALLOC_PERTURB_=165
 		export MALLOC_CHECK_ MALLOC_PERTURB_
+		if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
+		   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
+		   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
+		then
+			g=
+			LD_PRELOAD="libc_malloc_debug.so.0"
+			for t in \
+				glibc.malloc.check=1 \
+				glibc.malloc.perturb=165
+			do
+				g="${g#:}:$t"
+			done
+			GLIBC_TUNABLES=$g
+			export LD_PRELOAD GLIBC_TUNABLES
+		fi
 	}
 	teardown_malloc_check () {
 		unset MALLOC_CHECK_ MALLOC_PERTURB_
+		unset LD_PRELOAD GLIBC_TUNABLES
 	}
 fi