sci-electronics/ngspice: reenable tests

sci-electronics/ngspice: reenable tests

If USE=debug is not set, all tests should pass now - excessive output
which broke empty-1.cir and resume-1.cir was a side effect of enabling
NDEV, which we have just disabled as recommended by upstream.

If USE=debug *is* set tests cannot presently succeed because of excessive
output in all 4 tests in regression/lib-processing/. Just print a
warning and skip the tests altogether.

See-Also: https://sourceforge.net/p/ngspice/bugs/353/
Gentoo-Bug: https://bugs.gentoo.org/642460
Package-Manager: Portage-2.3.13, Repoman-2.3.3