Restore upstream sanity tests t/003safesys.t and t/013dbschema.t

[mrenvoize]

These two sanity tests exist in 5.2 but were dropped during Harmony's divergence. Restoring them re-adds cheap, orthogonal guardrails.

t/013dbschema.t (SQL reserved-word check):

• Adapted to Harmony's schema API: use the positional-driver factory Bugzilla::DB::Schema->new('Mysql') instead of upstream's Moo-style ->new(db => $fake_db), which Harmony does not have.
• Whitelisted ACTION, DATA, DOMAIN and TIMESTAMP, following upstream's existing practice of exempting words that are already used as table/ column names (VALUE, TYPE, ALIAS, COALESCE). These four are used by the AntiSpam, BugmailFilter, Push and RequestNagger tables and are safe on every supported backend because Bugzilla quotes identifiers.

t/003safesys.t (safe system/exec usage):

• Passes 653/653 after fixing one call in RequestNagger's send-request-nags.pl to pass the program and its arguments explicitly (system($command[0], @command[1 .. $#command])) so it satisfies the Support::Systemexec prototype instead of passing a single list.

t/001compile.t is intentionally NOT restored here: making it pass requires updating the shared t/Support/Files.pm to understand Harmony's Bugzilla::Extension:: module layout and to skip .disabled files and feature-gated modules. That is tracked as separate follow-up work.