fix: Decrease postgres test timeout (make test-postgres) (#3108)

This commit lowers the postgres test timeout from 30m to 20m, currently
our postgres tests seem to take 8-10m, a 2x factor should suffice.
Comments were updated in both places to reflect the reasoning and
necessity of keeping these values in sync.

They used to take longer but the `count` was lowered in
3d40cb85b7.

The actual timeout value of `make test-postgres` got overlooked in
https://github.com/coder/coder/pull/3079.

.github/workflows/coder.yaml

@@ -233,7 +233,10 @@ jobs:
   test-go-postgres:
     name: "test/go/postgres"
     runs-on: ubuntu-latest
-    # This timeout must be greater than go test -timeout.
+    # This timeout must be greater than the timeout set by `go test` in
+    # `make test-postgres` to ensure we receive a trace of running
+    # goroutines. Setting this to the timeout +5m should work quite well
+    # even if some of the preceding steps are slow.
     timeout-minutes: 25
     steps:
       - uses: actions/checkout@v3