Testing: fix deprecation warning and error in PhpStorm

[putyourlightson]

Fixes the deprecation warning (see http://phptest.club/t/bootstrap-deprecations-in-3-0/2196):

DEPRECATION: 'settings: bootstrap: _bootstrap.php' option is deprecated! Replace it with: 'bootstrap: _bootstrap.php' (not under settings section). See https://bit.ly/2YrRzVc 

And adds ob_start(); to the main _bootstrap.php file to prevent headers already sent errors being thrown when running tests through PhpStorm (https://stackoverflow.com/questions/31175636/headers-already-sent-running-unit-tests-in-phpstorm/55040543#55040543).

[codecov]

Codecov Report

Merging #4772 into develop will decrease coverage by 2.05%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             develop   #4772      +/-   ##
============================================
- Coverage      33.56%   31.5%   -2.06%     
+ Complexity     13548   12922     -626     
============================================
  Files            601     530      -71     
  Lines          40808   38680    -2128     
============================================
- Hits           13696   12186    -1510     
+ Misses         27112   26494     -618

Impacted Files	Coverage Δ	Complexity Δ
src/test/TestSetup.php	0% <0%> (ø)	44 <0> (-4)	⬇️
src/volumes/Temp.php	0% <0%> (-54.55%)	5% <0%> (ø)
src/elements/db/MatrixBlockQuery.php	0% <0%> (-44.88%)	28% <0%> (ø)
src/helpers/StringHelper.php	82.43% <0%> (-17.27%)	99% <0%> (-60%)
src/elements/db/GlobalSetQuery.php	50% <0%> (-10.87%)	6% <0%> (-2%)
src/helpers/Template.php	16.66% <0%> (-8.53%)	46% <0%> (-12%)
src/services/Path.php	47.18% <0%> (-7.05%)	58% <0%> (ø)
src/elements/MatrixBlock.php	0% <0%> (-6.46%)	47% <0%> (ø)
src/helpers/Image.php	82.14% <0%> (-5.54%)	53% <0%> (+1%)
src/db/Query.php	64.93% <0%> (-5.2%)	33% <0%> (ø)
... and 133 more

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[gtettelaar]

@angrybrad Should be ready for you to have a look at