neon_local: fix tenant create -c eviction_policy:... (#4004)

And add corresponding unit test.

The fix is to use `.remove()` instead of `.get()` when processing the
arugments hash map.
The code uses emptiness of the hash map to determine whether all
arguments have been processed.
This was likely a copy-paste error.

    
refs https://github.com/neondatabase/neon/issues/3942

control_plane/src/pageserver.rs

@@ -359,8 +359,8 @@ impl PageServerNode {
                 .transpose()
                 .context("Failed to parse 'trace_read_requests' as bool")?,
             eviction_policy: settings
-                .get("eviction_policy")
-                .map(|x| serde_json::from_str(x))
+                .remove("eviction_policy")
+                .map(serde_json::from_str)
                 .transpose()
                 .context("Failed to parse 'eviction_policy' json")?,
             min_resident_size_override: settings