regress: allow to specify directory containing expected files, for ecpg

regress: allow to specify directory containing expected files, for ecpg

The ecpg tests have their input directory in the build directory, as the tests
need to be built. Until now that required copying the expected/ directory to
the build directory in VPATH builds. To avoid needing to implement the same
for the meson build, add support for specifying the location of the expected
directory.

Now that that's not needed anymore, remove the copying of ecpg's expected
directory to the build directory in VPATH builds.

Author: Nazir Bilal Yavuz <[email protected]>
Author: Andres Freund <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/20220718202327[email protected]