Even if there are no lines covered in the file the gcov coverage report still
contains valueable information, the amount of uncovered lines and which exactly
they are. Set 'Covered="true"' for files we have a gcov report for even if they
have no lines covered. Otherwise CDash will neither show the uncovered line
count nor the detailed coverage report for this file.
When CTEST_EXTRA_COVERAGE_GLOB was used to collect otherwise uncovered files
'Covered="true"' was unconditionally set, so this can't be worse here.
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