Further fixes for tab-completion TAP tests.

Further fixes for tab-completion TAP tests.

Escape non-printable characters in failure reports, by using Data::Dumper
in Useqq mode.  Also, bump $Test::Builder::Level so the diagnostic
references the calling line, and use diag() instad of note(),
so it shows even in non-verbose mode (per request from Christoph Berg).

Also, give up on trying to test for the specific way that readline
chooses to overwrite existing text in the \DRD -> \drds test.
There are too many variants, it seems, at least on the libedit
side of things.

Dagfinn Ilmari Mannsåker and Tom Lane

Discussion: https://postgr.es/m/20200103110128.GA28967@msg.df7cb.de