test: deflake fast FFI buffer tests

The optimized buffer tests can leave compiler jobs running during
library teardown. Wait for those jobs before closing the libraries,
matching the existing integer validation tests.

Signed-off-by: Filip Skokan <panva.ip@gmail.com>
Assisted-by: Codex
PR-URL: #66327
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: panva

test/ffi/test-ffi-fast-buffer.js

@@ -95,6 +95,7 @@ test('optimized pointer arguments reject direct SharedArrayBuffers', () => {
 
     assert.throws(() => callFirstByte(shared), expected);
   } finally {
+    eval('%WaitForBackgroundOptimization()');
     lib.close();
   }
 });
@@ -202,6 +203,7 @@ test('optimized buffer signatures preserve pointer-like conversions', () => {
       }
     }
   } finally {
+    eval('%WaitForBackgroundOptimization()');
     lib.close();
   }
 });
@@ -235,6 +237,7 @@ test('multi-argument buffer signatures accept pointer BigInts', () => {
       assert.strictEqual(functions.sum_buffer(pointer, length), 28n);
     }
   } finally {
+    eval('%WaitForBackgroundOptimization()');
     lib.close();
   }
 });