tests/qtest: add TIMEOUT_MULTIPLIER

Some tests need more time when qemu is built with
"--enable-asan --enable-ubsan"

As was discussed here:
https://patchew.org/QEMU/20241112120100.176492-2-frolov@swemel.ru/

TIMEOUT_MULTIPLIER enviroment variable will be
a useful option, allowing non-invasive timeouts
increasing for a specific build.

Signed-off-by: Dmitry Frolov <frolov@swemel.ru>
Tested-by: Fabiano Rosas <farosas@suse.de>
[changed from ifndef to ?=]
Signed-off-by: Fabiano Rosas <farosas@suse.de>

scripts/mtest2make.py

@@ -27,7 +27,7 @@ def names(self, base):
 .speed.slow = $(foreach s,$(sort $(filter-out %-thorough, $1)), --suite $s)
 .speed.thorough = $(foreach s,$(sort $1), --suite $s)
 
-TIMEOUT_MULTIPLIER = 1
+TIMEOUT_MULTIPLIER ?= 1
 .mtestargs = --no-rebuild -t $(TIMEOUT_MULTIPLIER)
 ifneq ($(SPEED), quick)
 .mtestargs += --setup $(SPEED)