REGTESTS: add a regtest to validate various NTLM transitions

REGTESTS: add a regtest to validate various NTLM transitions

This test first performs two successive requests over the same
connection where reuse is expected, then perform two 401 which must
both work, testing both the transition from null->sess, and sess->sess.

This test could be backported to detect changes related to private
sessions.

Thanks to Omkhar Arasaratnam for the test.