Use saveXml() instead of save()

[duncan3dc]

As reported in issue #692 there seems to be an issue writing the XML code coverage to disk when using the \SoapClient class. This PR gets around that issue by using saveXml() to generate the XML and the file_put_contents() to write it

[codecov]

Codecov Report

Merging #721 into master will decrease coverage by 2.53%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #721      +/-   ##
============================================
- Coverage      85.1%   82.56%   -2.54%     
- Complexity      812      843      +31     
============================================
  Files            32       36       +4     
  Lines          2443     2529      +86     
============================================
+ Hits           2079     2088       +9     
- Misses          364      441      +77

Impacted Files	Coverage Δ	Complexity Δ
src/Report/Xml/Facade.php	91.53% <100%> (ø)	33 <0> (ø)	⬇️
src/Driver/Xdebug.php	8.82% <0%> (ø)	15% <0%> (?)
src/Driver/PHPDBG.php	10.34% <0%> (ø)	13% <0%> (?)
src/Driver/Driver.php	0% <0%> (ø)	0% <0%> (?)
src/Driver/PCOV.php	30% <0%> (ø)	3% <0%> (?)

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[sebastianbergmann]

@theseer Do you have an idea what could be going on here? Thanks!