riscv: kselftests: Fix mm build by removing testcases subdirectory

kselftests fails to build because the mm/testcases subdirectory is not
created and then the compiler fails to output the binary there.

So fix this by simply removing this subdirectory which is not very
useful.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20230915100113.13131-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

RISC-V: mm: Add tests for RISC-V mm

Add tests that enforce mmap hint address behavior. mmap should default
to sv48. mmap will provide an address at the highest address space that
can fit into the hint address, unless the hint address is less than sv39
and not 0, then it will return a sv39 address.

These tests are split into two files: mmap_default.c and mmap_bottomup.c
because a new process must be exec'd in order to change the mmap layout.
The run_mmap.sh script sets the stack to be unlimited for the
mmap_bottomup.c test which triggers a bottomup layout.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20230809232218.849726-3-charlie@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>