Fix trivial OSS test failures

[mszabo-wikia]

Address some straightforward test failures in HHVM OSS:

• Gate tests for internal Meta extensions behind appropriate runif checks.
• Set -vPHP7.EngineExceptions=false for some tests that assume this, as this knob defaults to false in internal builds but to true in OSS.
• Skip tests that exercise fb_(un)serialize() outside internal builds, since FBSerialize was removed from HHVM OSS in D46525242.
• Remove a hardcoded fbcode reference.

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