ConcaveHull sfcgal test lost

[komzpa]

In ​https://github.com/postgis/postgis/commit/5a6b8a512fd9c72402520a167571276e4504852f a switch to sfcgal backend got lost.

Switch should be returned into its place, and probably \i concave_hull.sql.

After that probably ​https://trac.osgeo.org/postgis/ticket/4126 will show up once again.

[robe]

Just a note on my local after turning on sfcgal, still waiting for concave hull test to finish.

Almost tempted to turn off sfcgal inside the function

[robe]

In 16659:

put back old sfcgal test (except use \i, and have the query be what was in sfcgal one), the sfcgal one was different
Closes #4131
Revise ST_ConcaveHull to use ST_UnaryUnion(ST_Collect..) (cause I know sfcgal doesn't replace with it's own implementation / a bit of a cheat)
Take out some ST_Union calls I think are over compensating.
References #3638

[robe]

Forgot to mention I left in Komzpa's new crueler (realer) test for ST_ConcaveHull, but not bothering to have it run in SFCGAL (it might run reasonably now), but I called it

concavehull_hard