Add support for EXTRA_REGRESS_OPTS for meson

Hi,

We use EXTRA_REGRESS_OPTS to make sure the whole test suite passes with 
our extension loaded and since I prefer develop in meson over using 
autotools and make the lack of support for EXTRA_REGRESS_OPTS in meson
has bugged me for a while.

I have implemented support for it as an environment variable we read in 
the testwrap script instead of adding it as a configuration option to 
meson.build. The reason for this is that I do not like having to run 
"meson reconfigure" all the time plus that for the PG_TEST_EXTRA we 
ended up having to add an environment variable anyway.

To use this run e.g. the following:

   EXTRA_REGRESS_OPTS="--load-extension=pgcrypto" meson test

Question: Would it make sense to rename it to PG_REGRESS_EXTRA_OPTS or 
something similar while we already touch touch this code to make the 
various options easier to remember?

Andreas