Refactor test_escape.c for additional ways of testing.

Refactor test_escape.c for additional ways of testing.

Start the file with static functions not specific to pe_test_vectors
tests.  This way, new tests can use them without disrupting the file's
layout.  Change report_result() PQExpBuffer arguments to plain strings.
Back-patch to v13 (all supported versions), for the next commit.

Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Backpatch-through: 13
Security: CVE-2025-4207