[Fortran/gfortran] Skip test that requires -fcheck=pointer

This test expects to fail with an error because you pass
an empty optional to a function where the argument is
not optional (and/or you're passing a pointer as an integer
I'm not sure which).

A reduced test case is:
```
program test
  call sub1c()
contains
  subroutine sub1c(a)
   integer, pointer,optional :: a
   call sub3(a) ! << INVALID
  end subroutine sub1c
  subroutine sub3(b)
    integer :: b
  end subroutine
end
```

gfortran compiles this and this happens when you run the binary:
Fortran runtime error: Pointer actual argument 'a' is not associated or not present

The test suite filters out `-f` options Flang doesn't support and until now,
Flang would produce code that somehow didn't crash at -O3. Since
https://lab.llvm.org/buildbot/#/builders/143/builds/3126 the code crashes.

llvm/llvm-project#113949 is the obvious suspect there
but the code crashes even when that is reverted. Perhaps there is some
non-determinism in play.

That said, the program is doing bad things so I think crashing is not
unexpected here. gofortran's code at -O0 also crashes.

There are other tests with -fcheck=pointer but I am just looking
to get the bots green so I've left the other ones alone.

Fortran/gfortran/regression/DisabledFiles.cmake

@@ -1780,6 +1780,9 @@ file(GLOB FAILING_FILES CONFIGURE_DEPENDS
   bounds_check_17.f90
   pr48958.f90
 
+  # Require -fcheck=pointer.
+  pointer_check_6.f90
+
   # These files require the __truncsfbf2 intrinsic that is not available
   # before GCC 13. Alternatively, it requires compiler-rt to be built and a
   # command line option provided to instruct the compiler to use it. Currently,