Rename system_tests.h -> smoke_tests.sh

Before adding more comprehensive 'acceptance tests', rename the existing
system-level tests to 'smoke tests' as they are supposed to be quick
tests of the most critical functionality.

Test Plan: $ ./precommit.sh
testbed/arcyd/savemail: fix accidental truncation

Don't overwrite all the saved mails with a blank line at the end
of savemail.

Test Plan: echo hi | ./savemail && cat savemail.txt