tests: Simplify 'substitute-binary' tests; reduce use of global variables.

* tests/substitute-binary.scm (signature-body): Change 'str' parameter
  to 'bv', and expect it to be a bytevector.
  (%signature-body, %wrong-signature, %acl): Remove.
  (signature): Rename to...
  (signature-field): ... this.  Add 'bv-or-str' parameter.  Change 'str'
  parameter to #:version.  Add #:public-key parameter.  Call
  'signature-body' directly.  Change domain part of the signature to
  'example.gnu.org'.
  ("not a number", "wrong version number", "valid
  narinfo-signature->canonical-sexp"): Use 'signature-field' instead of
  'signature' or %SIGNATURE.
  (test-error-condition): Add 'message-rx' parameter and honor it.
  ("corrupt signature data", "unauthorized public key", "invalid
  signature"): Adjust accordingly.
  (narinfo, %signed-narinfo): Remove.
  ("query narinfo with invalid hash"): Use '%narinfo' and
  'signature-field' instead of 'narinfo' and '%signature'.
  ("query narinfo signed with authorized key", "query narinfo signed
  with unauthorized key", "substitute, invalid hash", "substitute,
  unauthorized key"): Likewise.