tests: reduce trans_spill_threshold

It will allow to run 007_add_stop_node.pl on small virtual machines.
It seems like tests are passing even with trans_spill_threshold=10MB,
but lets be less extreme.

Author: funny-falcon

Cluster.pm

@@ -171,7 +171,7 @@ sub init
 			# For add_stop node test we need at least 4 * 3 * (2 * trans_spill_threshold)
 			# MB of shmem, and some bf members have only 2GB /dev/shm, so be
 			# careful upping this.
-			multimaster.trans_spill_threshold = 50MB
+			multimaster.trans_spill_threshold = 20MB
 
 			multimaster.binary_basetypes = ${binary_basetypes}