test: resolve flakiness in worker*.test-free-called

Fixes: nodejs#51003

The issue arises from the `getFreeCallCount()` function yielding the
initial value of 0. Upon instantiation of the `Worker` object, it
increments to 1. In the case of this flaky test, if the creation of the
`Worker` object is faster, the subsequent `getFreeCallCount()` call
always returns 1 instead of the expected 0.

test/addons/worker-buffer-callback/test-free-called.js

@@ -6,12 +6,14 @@ const { Worker } = require('worker_threads');
 const binding = path.resolve(__dirname, `./build/${common.buildType}/binding`);
 const { getFreeCallCount } = require(binding);
 
+// getFreeCallCount initial value is 0
+assert.strictEqual(getFreeCallCount(), 0);
+
 // Test that buffers allocated with a free callback through our APIs are
 // released when a Worker owning it exits.
 
 const w = new Worker(`require(${JSON.stringify(binding)})`, { eval: true });
 
-assert.strictEqual(getFreeCallCount(), 0);
 w.on('exit', common.mustCall(() => {
   assert.strictEqual(getFreeCallCount(), 1);
 }));