Make kconfig_compiler autotests use the KCONFIG_ADD_KCFG_FILES

Summary:
Instead of having an odd fake of it. Will help some required
refactorings and already showed some issues, fixed by this patch,
namely:
- don't use string(regex replace) to extract a string from another
string. in case it doesn't match it will offer the whole content which
is never what we want.
- messages(ERROR), the correct parameter is FATAL_ERROR, cmake
understands "ERROR" as mere output string
- turn the macro into a function, otherwise 2 calls in the same
subdirectory are dangerous.

CCBUG: 371562

Test Plan: tests still pass, projects that use the macro still build

Reviewers: #frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure

Differential Revision: https://phabricator.kde.org/D3178

0 files changed, 0 insertions, 0 deletions