[Flang][test] Fix Windows buildbot.

Add

    REQUIRES: shell

to tests that execute a UNIX shell script to not run on Windows.

flang/test/Evaluate/folding20.f90

@@ -1,4 +1,5 @@
 ! RUN: %S/test_folding.sh %s %t %flang_fc1
+! REQUIRES: shell
 ! Tests intrinsic MAXVAL/MINVAL function folding
 module m
   logical, parameter :: test_imaxidentity = maxval([integer::]) == -huge(0) - 1

flang/test/Semantics/call19.f90

@@ -1,4 +1,5 @@
 ! RUN: %S/test_errors.sh %s %t %flang_fc1
+! REQUIRES: shell
 ! Ensures that things that aren't procedures aren't allowed to be called.
 module m
   integer :: i

flang/test/Semantics/omp-allocate08.f90

@@ -1,4 +1,5 @@
 ! RUN: %S/test_errors.sh %s %t %flang_fc1 -fopenmp
+! REQUIRES: shell
 ! OpenMP Version 5.0
 ! 2.11.3 allocate Directive
 ! If list items within the ALLOCATE directive have the SAVE attribute, are a common block name, or are declared in the scope of a

flang/test/Semantics/omp-nested-barrier.f90

@@ -1,4 +1,5 @@
 ! RUN: %S/test_errors.sh %s %t %flang_fc1 -fopenmp
+! REQUIRES: shell
 ! OpenMP Version 4.5
 ! Various checks with the nesting of BARRIER construct