FindGTest: Optionally detect sources automatically (#14775)

Teach GTEST_ADD_TESTS an "AUTO" option to read the source files from the
SOURCES target property of an executable.
Help: Add FindGTest-AUTO-SOURCES topic release notes
Merge topic 'FindGTest-AUTO-SOURCES' into next

ac42bca8 Help: Add FindGTest-AUTO-SOURCES topic release notes
c50325b8 FindGTest: Optionally detect sources automatically (#14775)