refactor: modernize specs for prettiest colors from report formatter.

* specs/ext_yaml_emitter_spec.yaml, specs/ext_yaml_parser_spec.yaml,
specs/ext_yaml_scanner_spec.yaml, specs/lib_lyaml_spec.yaml: Use
`to_` matchers rather than `should_` matchers throughout.
Use specify:describe:context:it hierarchy consistently throughout.
Remove redundant `require "spec_helper"` blocks.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>