test-migrations: Fix shellcheck warnings.
test-migrations: Fix shellcheck warnings.

In tools/test-migrations line 18:
    echo "$new_auto_named_migrations" | sed 's/\[[x ]\] /  /'
    ^-- SC2001: See if you can use ${variable//search/replace} instead.

In tools/test-migrations line 27:
    echo 'ERROR: Migrations are not consistent with models!  Fix with `./tools/renumber-migrations`.'
         ^-- SC2016: Expressions don't expand in single quotes, use double quotes for that.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

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