[flang]Fix individual tests with lit when building out of tree

Summary:

Fix individual check tests with lit when building out-of-tree
`ninja check-flang-<folder>` was not working.
The CMakeLists.txt was looking for the lit tests in the source directory
instead of the build directory.

This commit extends @CarolineConcatto previous patch[D81002]

Reviewers: DavidTruby, sscalpone, tskeith, CarolineConcatto, jdoerfert

Reviewed By: DavidTruby

Subscribers: flang-commits, llvm-commits, CarolineConcatto

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D82120