Add bug link to test_fcoverage_mapping wasm64 issue. NFC (#26912)

Author: sbc100

test/test_core.py

@@ -9815,7 +9815,7 @@ def test_modularize_instance_embind(self):
     self.assertContained('main\nfoo\nbar\n', self.run_js('runner.mjs'))
 
   @no_esm_integration('fcoverage is not compatible with WASM_ESM_INTEGRATION')
-  @no_wasm64('TODO: fcoverage in memory64')
+  @no_wasm64('https://github.com/emscripten-core/emscripten/issues/26865')
   @no_wasm2js('wasm binary required to produce code coverage results with llvm-cov')
   def test_fcoverage_mapping(self):
     expected = '''\