test: Rewrite JWT cache tests

Timing dependent tests in the IO tests don't work too well when the next
commit increases the JWT parsing performance.

The remaining IO tests are for coverage and basic breakage. Loadtests
are adapted so that performance regressions for JWT caching would be
detected that way.

nix/tools/loadtest.nix

@@ -55,6 +55,8 @@ let
         export PGRST_DB_POOL="1"
         export PGRST_DB_TX_END="rollback-allow-override"
         export PGRST_LOG_LEVEL="crit"
+        export PGRST_JWT_SECRET="reallyreallyreallyreallyverysafe"
+        export PGRST_JWT_CACHE_MAX_LIFETIME="86400"
 
         mkdir -p "$(dirname "$_arg_output")"
         abs_output="$(realpath "$_arg_output")"