Disable Xdebug during tests on Travis

[johnbillion]

My local installation was running its tests 2x slower than I expected, and I narrowed down the problem to the Xdebug extension being unexpectedly enabled on the CLI. Disabling Xdebug restored the test suite speed back to its previous level.

I did some research and it turns out Xdebug can be disabled on Travis. Refs:

• ​https://docs.travis-ci.com/user/languages/php#Disabling-preinstalled-PHP-extensions
• ​https://github.com/travis-ci/travis-ci/issues/444
• ​https://github.com/travis-ci/travis-ci/issues/5780

It would be worth pushing a change which disables Xdebug for core to see if it speeds things up (it should) and to see if it breaks anything (it shouldn't).

[johnbillion]

In 40138:

Build/Test Tools: Disable Xdebug when testing on Travis.

This should have some performance benefits, but I guess we'll need to wait and see.

See #39978

[johnbillion]

This worked better than I expected.

Total time on Travis has dropped from ~2h30 to ~1h30, and the actual build test time has dropped from ~40 mins to ~24 mins.

Need to fix the error that it's causing on hhvm and php nightly.

Ref: ​https://travis-ci.org/aaronjorbin/develop.wordpress/builds

[johnbillion]

In 40139:

Build/Test Tools: Only attempt to disable Xdebug on stable builds of PHP.

Unstable builds of PHP (nightly and hhvm) don't have the Xdebug extension enabled. Attempting to disable it results in a build error.

See #39978

[johnbillion]

I think this is worth back-porting to the branches to speed up branch builds.

[johnbillion]

In 40225:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40226:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40227:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40228:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40229:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40230:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40231:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40232:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40233:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978

[johnbillion]

In 40234:

Build/Test Tools: Disable Xdebug when testing on Travis to increase performance.

See #39978