pgsql: Add tests for errors during SSL or GSSAPI handshake

Add tests for errors during SSL or GSSAPI handshake

These test that libpq correctly falls back to a plaintext connection
on handshake error, in the "prefer" modes.

Reviewed-by: Michael Paquier
Discussion:
https://www.postgresql.org/message-id/CAOYmi%2Bnwvu21mJ4DYKUa98HdfM_KZJi7B1MhyXtnsyOO-PB6Ww%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ef7fa900fb587cbaac9f3e943f789155b97aa02b

Modified Files
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src/backend/libpq/be-secure-gssapi.c               |  3 +++
src/backend/libpq/be-secure.c                      |  3 +++
src/interfaces/libpq/t/005_negotiate_encryption.pl | 20 ++++++++++++++++++++
3 files changed, 26 insertions(+)