travis-ci: Force object destructors to run under HHVM

The RequestContextTest::testImportScopedSession test will fail if object
destructors are not run when an object is destroyed. HHVM does not
enable this behavior by default to improve performance. Travis-ci does
not currently have an easy method to set custom ini configuration for
the hhvm-nightly test runner, so instead we will use a custom test
runner script and add command line parameters to enable the desired HHVM
behavior.

Bug: 73177