test-backend: Change --parallel default to use `cpu_count`.
test-backend: Change --parallel default to use `cpu_count`.

The number of processes to run the backend tests is currently a
hardcoded value, this commit transistions the default to be based on the
number of logical CPUs available.

synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,tommyip/zulip,punchagan/zulip,shubhamdhama/zulip,tommyip/zulip,kou/zulip,rht/zulip,hackerkid/zulip,tommyip/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,andersk/zulip,timabbott/zulip,zulip/zulip,brainwane/zulip,rishig/zulip,brainwane/zulip,shubhamdhama/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,rishig/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,brainwane/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,shubhamdhama/zulip,tommyip/zulip,rht/zulip,kou/zulip,kou/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,rishig/zulip,synicalsyntax/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,rht/zulip,hackerkid/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,hackerkid/zulip,kou/zulip,showell/zulip,tommyip/zulip,hackerkid/zulip,zulip/zulip,rishig/zulip,zulip/zulip,timabbott/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,eeshangarg/zulip,rishig/zulip,hackerkid/zulip,tommyip/zulip,zulip/zulip,zulip/zulip,showell/zulip,shubhamdhama/zulip,rishig/zulip,brainwane/zulip,shubhamdhama/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip,andersk/zulip,rht/zulip,andersk/zulip,timabbott/zulip,shubhamdhama/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,timabbott/zulip,eeshangarg/zulip,andersk/zulip,tommyip/zulip,punchagan/zulip,kou/zulip,andersk/zulip,rishig/zulip,synicalsyntax/zulip