tests: Use invoke and return #t from all builders.

tests: Use invoke and return #t from all builders.

* tests/packages.scm ("package-source-derivation, snippet", "trivial")
("trivial with local file as input", "trivial with source")
("trivial with system-dependent input", "trivial with #:allowed-references")
("--search-paths with pattern", "--search-paths with single-item search path")
("replacement also grafted"): In the builders, raise an exception on errors
and otherwise return #t.  Use invoke.