In commit 7a969fe21d (cmMakefile: Refactor API to better handle empty
config values, 2020-06-30, v3.19.0-rc1~567^2), calls to
`GetGeneratorConfigs` that pass `OnlyMultiConfig` only want to get any
configurations listed if the generator is multi-config. Fix the
implementation to actually do that.
Fixes: #21316
Revert commit v3.8.0-rc1~305^2 (Remove CTestTestfile.cmake when BUILD_TESTING
is OFF, 2016-11-14) again. We reverted it once in commit v3.8.0-rc3~22^2
(Revert "Remove CTestTestfile.cmake when BUILD_TESTING is OFF", 2017-03-06) but
it was accidentally restored by commit v3.11.0-rc1~387^2 (server: add
"ctestInfo" request to get test info, 2017-10-25), perhaps due to conflict
resolution during rebase.
We cannot remove `CTestTestfile.cmake` when testing is off because it breaks
projects that never enable testing but create their own `CTestTestfile.cmake`
manually instead. Revert the change again and add a test case.
Do not use PROJECT_BINARY_DIR before it is defined. If it is not
defined when needed, fail with an error message suggesting that the
project() command be invoked first.
