Initialize enrollment preparations of fake attestation client to true

The default failure should not happen because of unprepared attestation;
instead it should controlled by thw allowlisting mechanism. Otherwise,
the retry logic could cause long testing time or even timeout.

As for testing, chromeos:chromeos_unittests and all affected
browser_tests are passed.

Bug: 1107426
Change-Id: Ie23d2618069b0511323d0f686a64ddfe0e05a054
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308625Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Leo Lai <cylai@google.com>
Cr-Commit-Position: refs/heads/master@{#790343}

chromeos/dbus/attestation/fake_attestation_client.h

@@ -101,7 +101,7 @@ class COMPONENT_EXPORT(CHROMEOS_DBUS_ATTESTATION) FakeAttestationClient
   AttestationClient::TestInterface* GetTestInterface() override;
 
  private:
-  bool is_prepared_{false};
+  bool is_prepared_ = true;
   std::deque<bool> preparation_sequences_;
 
   bool is_enrolled_ = false;