Make group/rc and common/rc sources explicit and reenable SC2034

Make group/rc and common/rc sources explicit and reenable SC2034

SC2034 (unused variable) is a useful warning, but we disabled it because
we set a bunch of global variables as test metadata. However, this is
easy to work around: as Bart demonstrated, an echo "$VAR" > /dev/null
does the trick.

However, we don't want to copy-and-paste this everywhere, so we need to
source something everywhere. Bart's idea was to put this in
common/shellcheck and source that everywhere. Sourcing a file just to
appease the linter is silly, though, so instead, this adds explicit
sources of tests/*/rc to each test, which in turn sources common/rc,
which in turn sources common/shellcheck.

Signed-off-by: Omar Sandoval <osandov@fb.com>