CI: Split tests in two groups by backend

[datapythonista]

Testing if splitting tests in this naive way solves the disk space problem. I guess docker images for the unused backends should not be download and storage consumption should be much less.

[datapythonista]

@jreback this solves the disk space problem by splitting the tests in two groups of backends. All green, except the conda build build, which is a separate issue (see #2195).

I couldn't get this working with spark and omnisci, so those two are not being tested here (I'll create issues to add them back if we merge this).

[jreback]

yeah create issues for those (can run as separate backends entirely)

will look soon

[jreback]

is there someway you can actually put the names rather than backends_1, e.g.
mysql-parquet-postgres-sqlite (e.g. more informative in the displays / code).

[jreback]

i merged the feedstock conda job so in theory this should be all green if you rebase

[datapythonista]

i merged the feedstock conda job so in theory this should be all green if you rebase

I changed the BACKEND_1... names. Let's see if we get a green build then.

[datapythonista]

LinuxCondaBuild is still failing, I think using Python 3.7 is needed, checking in #2202.

[jreback]

kk happy to merge this and can proceed?

[datapythonista]

Yes, I'd merge this, and then we see the conda build problem. I guess just the change in #2202 is needed (probably better to add the parameter, as @xmnlab did).

[jreback]

thanks @datapythonista