tests: Handle $RANDOM not being supported by the shell

In various places in our test makefiles and scripts we use the
shell $RANDOM to create a random number. This is a bash
specific extension, and doesn't work on other shells.
With dash the shell doesn't complain, it just effectively
always evaluates $RANDOM to 0:
  echo $((RANDOM + 32768))     => 32768

However, on NetBSD the shell will complain:
  "-sh: arith: syntax error: "RANDOM + 32768"

which means that "make check" fails.

Switch to using "${RANDOM:-0}" instead of $RANDOM,
which will portably either give us a random number or zero.
This means that on non-bash shells we don't get such
good test coverage via the MALLOC_PERTURB_ setting, but
we were already in that situation for non-bash shells.

Our only other uses of $RANDOM (in tests/qemu-iotests/check
and tests/qemu-iotests/162) are in shell scripts which use
a #!/bin/bash line so they are always run under bash.
Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NKamil Rytarowski <n54@gmx.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1500029117-6387-1-git-send-email-peter.maydell@linaro.org

tests/Makefile.include

@@ -834,7 +834,7 @@ $(patsubst %, check-qtest-%, $(QTEST_TARGETS)): check-qtest-%: $(check-qtest-y)
 	$(if $(CONFIG_GCOV),@rm -f *.gcda */*.gcda */*/*.gcda */*/*/*.gcda,)
 	$(call quiet-command,QTEST_QEMU_BINARY=$*-softmmu/qemu-system-$* \
 		QTEST_QEMU_IMG=qemu-img$(EXESUF) \
-		MALLOC_PERTURB_=$${MALLOC_PERTURB_:-$$((RANDOM % 255 + 1))} \
+		MALLOC_PERTURB_=$${MALLOC_PERTURB_:-$$(( $${RANDOM:-0} % 255 + 1))} \
 		gtester $(GTESTER_OPTIONS) -m=$(SPEED) $(check-qtest-$*-y) $(check-qtest-generic-y),"GTESTER","$@")
 	$(if $(CONFIG_GCOV),@for f in $(gcov-files-$*-y) $(gcov-files-generic-y); do \
 	  echo Gcov report for $$f:;\
@@ -845,7 +845,7 @@ $(patsubst %, check-qtest-%, $(QTEST_TARGETS)): check-qtest-%: $(check-qtest-y)
 $(patsubst %, check-%, $(check-unit-y) $(check-speed-y)): check-%: %
 	$(if $(CONFIG_GCOV),@rm -f *.gcda */*.gcda */*/*.gcda */*/*/*.gcda,)
 	$(call quiet-command, \
-		MALLOC_PERTURB_=$${MALLOC_PERTURB_:-$$((RANDOM % 255 + 1))} \
+		MALLOC_PERTURB_=$${MALLOC_PERTURB_:-$$(( $${RANDOM:-0} % 255 + 1))} \
 		gtester $(GTESTER_OPTIONS) -m=$(SPEED) $*,"GTESTER","$*")
 	$(if $(CONFIG_GCOV),@for f in $(gcov-files-$(subst tests/,,$*)-y) $(gcov-files-generic-y); do \
 	  echo Gcov report for $$f:;\