tests/tcg: target/mips: Change directory name 'bit-counting' to 'bit-count'

Change directory name 'bit-counting' to 'bit-count'. This is just for
cosmetic and consistency sake. This was the only subdirectory in MSA
test directory that uses ending 'ing'.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>