Do not run tests in parallel, default MAX_CASES to 1

@michaelklishin:
> Some test cases cannot be run in parallel since they rely on target
> node state as a shared resource.

If/when we want to improve this:

@michaelklishin:
> What we can do is make this configurable. Some tests can run in
> parallel, e.g. all rabbitmq-diagnostics tests that do not trigger/report
> alarms. But this only would be useful for interactive runs or if we
> split all tests into groups and allow parallel runs for some groups.

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

Makefile

@@ -10,7 +10,7 @@ DEP_EARLY_PLUGINS = rabbit_common/mk/rabbitmq-early-plugin.mk
 DEP_PLUGINS = rabbit_common/mk/rabbitmq-plugin.mk
 
 VERBOSE_TEST ?= true
-MAX_CASES ?= $(shell nproc)
+MAX_CASES ?= 1
 
 MIX_TEST = mix test --max-cases=$(MAX_CASES)