Ostensibly fix all flaky tests

backend/src/langs.ts

@@ -1650,7 +1650,7 @@ main :: Effect Unit
 main = do
   log "Hello, world!"
 `,
-    timeout: 45,
+    timeout: 60,
   },
   python: {
     aliases: ["python3", "python2", "py"],

backend/src/test-runner.ts

@@ -11,7 +11,7 @@ import * as api from "./api";
 import { LangConfig, langs } from "./langs";
 
 const TIMEOUT_SECS = 5;
-const CONCURRENCY = 16;
+const CONCURRENCY = 2;
 
 function findPosition(str: string, idx: number) {
   const lines = str.substring(0, idx).split("\n");