test: reset sql.Promise after custom promise library test

The 'custom promise library' test sets sql.Promise to a FakePromise
class but never restores it. This leaks into subsequent tests that
rely on shared.Promise being the native Promise, causing failures
when tests use _poolValidate or other code paths that create promises
via the shared module.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dhensby

test/common/unit.js

@@ -123,6 +123,7 @@ describe('Unit', () => {
     sql.Promise = FakePromise
     sql.close().then(() => {
       assert.strictEqual(resolved, true)
+      sql.Promise = Promise
       done()
     })
   })