tests: Rework markup parsing test to not stop on first failure

Previously, the markup parsing test would load a given markup file and
try to parse it several ways. It would return as soon as one of the
attempts failed — meaning that bugs only seen with non-nul-terminated,
or differently chunked, parse runs could never be caught.

Rework the tests so that all markup files are tested all ways, and we
assert that all ways of parsing them give the same result.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
8 jobs for ossfuzz-9672-markup-overflow in 19 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #133159
cross-android_api21_arm64

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passed #133160
cross-android_api28_arm64

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passed #133161
cross-mingw64

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passed #133158
fedora-autotools-x86_64

00:05:55

passed #133157
fedora-x86_64

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passed #133162
win32
msys2-mingw32

00:08:15

passed #133163
win32
vs2017-x64

00:07:13

 
  Coverage
passed #133164
coverage

00:01:58

74.9%