test: make test-memory-usage predictable

The current test relies on undeterministic behavior from V8 GC, and on
newer versions of V8 this test ocasionally fails because that behavior
changed.  Prevent that from happening using --predictable-gc-schedule.
If this test fails in the future, it should fail consistently instead of
ocasionally, which should help debug.

Ref: nodejs/node-v8#144 (comment)
Signed-off-by: Matheus Marchini <mmarchini@netflix.com>

test/parallel/test-memory-usage.js

@@ -19,7 +19,7 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
-// Flags: --no-concurrent-array-buffer-freeing
+// Flags: --predictable-gc-schedule
 'use strict';
 const common = require('../common');
 const assert = require('assert');