Fix "PhutilOpaqueEnvelopeTestCase" under PHP 7.4 with "zend.exception_ignore_args"

Summary:
See PHI1894. PHP 7.4 introduced a new runtime configuration option, "zend.exception_ignore_args", which removes the "args" from exception backtraces.

The "PhutilOpaqueEnvelopeTestCase" relies on this behavior (since it explicitly inspects stack frames). Although the test isn't critical and could be restructured, it seems like there is little value to ever enabling this option in the context of Phabricator.

Disable it at startup so environments are more consistent across different PHP versions and configurations.

Test Plan:

• Enabled "zend.exception_ignore_args" under PHP 7.4.
• Ran "PhutilOpaqueEnvelopeTestCase".
• Before: failure, expected signpost value not present in stack trace (because no "args" are present on the exception).
• After: test passes.

Differential Revision: https://secure.phabricator.com/D21473