Ignore more environment variables in TAP tests

Ignore more environment variables in TAP tests

Various environment variables were not getting reset in the TAP tests,
which would cause failures depending on the tests or the environment
variables involved.  For example, PGSSL{MAX,MIN}PROTOCOLVERSION could
cause failures in the SSL tests.  Even worse, a junk value of
PGCLIENTENCODING makes a server startup fail.  The list of variables
reset is adjusted in each stable branch depending on what is supported.

While on it, simplify a bit the code per a suggestion from Andrew
Dunstan, using a list of variables instead of doing single deletions.

Reviewed-by: Andrew Dunstan, Daniel Gustafsson
Discussion: https://postgr.es/m/YLbjjRpucIeZ78VQ@paquier.xyz
Backpatch-through: 9.6