specs: no need to explicitly require spec_helper any more.

Since Specl 11, every spec-file automatically loads the
spec_helper.lua from the same directory, if any.
* specs/container_spec.yaml, specs/debug_spec.yaml,
specs/functional_spec.yaml, specs/io_spec.yaml,
specs/list_spec.yaml, specs/math_spec.yaml,
specs/object_spec.yaml, specs/optparse_spec.yaml,
specs/package_spec.yaml, specs/set_spec.yaml,
specs/strbuf_spec.yaml, specs/string_spec.yaml,
specs/table_spec.yaml, specs/tree_spec.yaml: Remove explicit
`require "spec_helper"`.

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