Improve ext.test semantics, avoid OpenBSD sh errexit issue. * tests/ext.test: Inside shell compound command, use `if $cmd; then Exit 1; fi' rather than `$cmd && Exit 1', to fix failure with OpenBSD sh introduced with last patch. Actually ensure that a rule for .EXT.o is created for each known extension EXT. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>