Fix PHP 8.0 tests under ZTS

[johnpbloch]

It looks like the version of PHP on Travis was compiled with ZTS mode turned on and the tests are failing because the macro for the now undefined tsrm_set_interpreter_context function was replacing everything but the semicolon, and the semicolon was causing compilation errors.

So in this fix, instead of a macro I just used a function that returns a null pointer, so it should compile correctly now. I suppose we'll see if that's true once the travis tests run 😅

[coveralls]

Coverage increased (+1.02%) to 86.677% when pulling f05dc3f on johnpbloch:fix/php-8.0-tests into f055bcd on php-zookeeper:master.

[coveralls]

Coverage decreased (-0.8%) to 84.835% when pulling 90a1842 on johnpbloch:fix/php-8.0-tests into f055bcd on php-zookeeper:master.

[johnpbloch]

🤦🏼‍♂️ looks like zookeeper just released zookeeper 3.7 in the last few weeks, so now 3.5 is no longer in the standard apache mirror setup. I'll switch 3.5 to build from archive.apache.org instead, but it's probably worthwhile to also test on 3.7 eventually.

[johnpbloch]

I'm going to close this PR. I've spent a bit of time working through some of the TSRM internals and the complexity is way beyond what I'm able to handle at the moment. I can get the extension to compile in ZTS builds of PHP 8, but it still segfaults for those 3 tests (watcher-related tests). At this point, I think the TSRM code really does need to be migrated to the 7.x way of handling those resources.