Tests: Fix ConfigSources test with empty CMAKE_BUILD_TYPE

Brad Kingrequested to merge
brad.king/cmake:test-ConfigSources-config into master

The test requires a non-empty CMAKE_BUILD_TYPE to function on single-config generators. Force a non-empty configuration if the test is configured explicitly with empty -DCMAKE_BUILD_TYPE=.

Backport: release

Edited by Brad King

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