Add LIBXML_NOBLANKS to make the output same on Linux and Windows

[mhujer]

We are using this library through Symfony to inline CSS in e-mails. We are also
using snapshot testing for rendered e-mails templates and I discovered that the
output of this library differs between Windows (dev machine) and Linux (CI).

I tried running the test suite on Windows and the test
CssToInlineStylesTest::testSpecificity() fails because of whitespace difference:
on Windows there is an extra \n before the closing </a> in the output.

I tried loading and saving a a super-simplified HTML <a>\n<img>\n</a>
through the DOMDocument without this library but with the same settings
and a similar thing happen (newline before is present on Windows but missing
on Linux).

When the option LIBXML_NOBLANKS is added to loadHTML() the output no longer
differs between the Windows and Linux.

To be honest, I'm not sure if this is a proper fix.
It should be rather considered a bug report with a workaround.

[mhujer]

Travis php-nightly fails for unrelated reason: PHP Fatal error: Uncaught SebastianBergmann\CodeCoverage\RuntimeException: xdebug.coverage_enable=On has to be set in php.ini

[stof]

The error seems related to php nightly using Xdebug 3 on Travis.

[stof]

CssToInlineStylesTest::testSpecificity was failing on Linux for me as well (and on the CI), so I fixed the expectation to have a \n in it.

[stof]

@mhujer I see that you also updated that PR. But please explain again what it is about. #224 is the proof that adding LIBXML_NOBLANKS is not necessary to make the testsuite pass on both Windows and Linux, because it is already the case without it.

[mhujer]

@stof There is still something I need to investigate in our test suite, because the tests there pass on Windows and fail in CI on Linux, but both environments pass when I use my branch with the LIBXML_NOBLANKS. So maybe it is issue just with some more complex HTML.

I'd like to keep the PR open for now at least until I investigate what the issue is.

[mhujer]

Surprisingly, the PHP behaviour changed between 8.0.5 and 8.0.14, so the output is same on Windows and Linux even without this patch.