Tests: validate CONFIG REWRITE for all params. (#7764)

This is a catch-all test to confirm that that rewrite produces a valid
output for all parameters and that this process does not introduce
undesired configuration changes.

(cherry picked from commit a8b7268911)

tests/unit/introspection.tcl

@@ -134,4 +134,28 @@ start_server {tags {"introspection"}} {
 
         }
     }
+
+    # Do a force-all config rewrite and make sure we're able to parse
+    # it.
+    test {CONFIG REWRITE sanity} {
+        # Capture state of config before
+        set configs {}
+        foreach {k v} [r config get *] {
+            dict set configs $k $v
+        }
+
+        # Rewrite entire configuration, restart and confirm the
+        # server is able to parse it and start.
+        assert_equal [r debug config-rewrite-force-all] "OK"
+        restart_server 0 0
+        assert_equal [r ping] "PONG"
+
+        # Verify no changes were introduced
+        dict for {k v} $configs {
+            assert_equal $v [lindex [r config get $k] 1]
+        }
+    }
+
+    # Config file at this point is at a wierd state, and includes all
+    # known keywords. Might be a good idea to avoid adding tests here.
 }