Convert bin/sh over to ATF
The sed call isn't needed
Apply review comments.

1. Localize the TEST_METADATA var in Makefile.inc
2. SUBDIR -> TEST_SUBDIRS. That way kyua will iterate down subdirectories
[1].

Submitted-by: pho [22]
Remove TEST_METADATA.functional_test

This isn't required and seems to trigger 3 failures:
- mail1.0
- mail2.0
- read2.0