test: /var a tmpfs to prevent tests from trampling on each other

Multiple parallel tests writing to the same /var leads to quite
unexpected results.  Let's start fresh in each test instead.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/skel/etc/fstab

@@ -3,6 +3,7 @@ tmpfs		/tmp		tmpfs	mode=1777		0	0
 mkdir#-p#-m0755	/tmp/etc.u	helper	none			0	0
 mkdir#-p#-m0755	/tmp/etc.w	helper	none			0	0
 tmpfs		/run		tmpfs	mode=0755,nosuid,nodev	0	0
+tmpfs		/var		tmpfs	mode=0777		0	0
 
 # Each test needs a dedicated /etc for finit.conf etc.
 etc /etc overlay lowerdir=/etc,upperdir=/tmp/etc.u,workdir=/tmp/etc.w	0	0