COMPMID-3329: failures in nightly (GC/GEMM and CPP/DFT)

- Can't recreate GC/GEMM
- Fix on CPP/DFT/DFT2D/Complex using the appropriate tolerance
- Fix CPP/DFT/Conv/Real2Real by removing concurrency issue on faulty function

Looking at the proper fix for CPP/DFT/Conv/Real2Real since it is a concurrency issue

Change-Id: Ic846dc7925d4c1b47e14181656c905f90778cdeb
Signed-off-by: Manuel Bottini <manuel.bottini@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/3085
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
diff --git a/tests/validation/reference/DFT.cpp b/tests/validation/reference/DFT.cpp
index ae030c7..7221312 100644
--- a/tests/validation/reference/DFT.cpp
+++ b/tests/validation/reference/DFT.cpp
@@ -269,9 +269,7 @@
 
     // MemSet dst memory to zero
     std::memset(dst.data(), 0, dst.size());
-#if defined(_OPENMP)
-    #pragma omp parallel for collapse(5)
-#endif /* _OPENMP */
+
     for(uint32_t b = 0; b < N; ++b)
     {
         for(uint32_t co = 0; co < Co; ++co)