Fix flakiness in std file_server tests (#5306)

Fixes: #5275

std/http/file_server_test.ts

@@ -28,9 +28,16 @@ async function startFileServer(): Promise<void> {
   assert(s !== null && s.includes("server listening"));
 }
 
-function killFileServer(): void {
+async function killFileServer(): Promise<void> {
   fileServer.close();
-  fileServer.stdout?.close();
+  // Process.close() kills the file server process. However this termination
+  // happens asynchronously, and since we've just closed the process resource,
+  // we can't use `await fileServer.status()` to wait for the process to have
+  // exited. As a workaround, wait for its stdout to close instead.
+  // TODO(piscisaureus): when `Process.kill()` is stable and works on Windows,
+  // switch to calling `kill()` followed by `await fileServer.status()`.
+  await Deno.readAll(fileServer.stdout!);
+  fileServer.stdout!.close();
 }
 
 test("file_server serveFile", async (): Promise<void> => {
@@ -46,7 +53,7 @@ test("file_server serveFile", async (): Promise<void> => {
     );
     assertEquals(downloadedFile, localFile);
   } finally {
-    killFileServer();
+    await killFileServer();
   }
 });
 
@@ -68,7 +75,7 @@ test("serveDirectory", async function (): Promise<void> {
       assert(/<td class="mode">(\s)*\(unknown mode\)(\s)*<\/td>/.test(page));
     assert(page.includes(`<a href="/README.md">README.md</a>`));
   } finally {
-    killFileServer();
+    await killFileServer();
   }
 });
 
@@ -81,7 +88,7 @@ test("serveFallback", async function (): Promise<void> {
     assertEquals(res.status, 404);
     const _ = await res.text();
   } finally {
-    killFileServer();
+    await killFileServer();
   }
 });
 
@@ -99,7 +106,7 @@ test("serveWithUnorthodoxFilename", async function (): Promise<void> {
     assertEquals(res.status, 200);
     _ = await res.text();
   } finally {
-    killFileServer();
+    await killFileServer();
   }
 });