If $HOME is not set (running from cron perhaps), the tests will output:
26d25
< ./comsub-posix.tests: line 103: cd: HOME not set
28d26
< ./comsub-posix.tests: line 106: cd: HOME not set
--- tests/comsub-posix.tests	2016-11-29 03:41:57.414081235 +0000
+++ tests/comsub-posix.tests	2016-11-29 03:36:30.425718528 +0000
@@ -1,4 +1,5 @@
-
+HOME=${HOME:-/}
+export HOME
 # works right
 echo ab$(echo mnop)yz
 # works right