Port CMake testhelpers away from deprecated globalBuildDirIndexKey()

The entry at Config::globalBuildDirIndexKey() is read only once - in
CMake::currentBuildDirIndex() - for backwards compatibility. This
function attempts to read an entry at Config::buildDirIndexKey() first.
Therefore, defaultConfigure() can safely write at this newer key,
assuming the current runtime does not change between the call to
defaultConfigure() and subsequent calls to CMake::currentBuildDirIndex()
(which is hopefully the case).

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1 job for !209 with cmake-const-config-entry-names in 34 minutes and 10 seconds (queued for 25 minutes and 14 seconds)
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passed #83929
linux kf5-qt5 qt5.15

00:34:10

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