selftests: find_symbol: Actually use load_mod() parameter

The parameter passed to load_mod() is stored in $MOD, but never used.
Obviously it was intended to be used instead of the hardcoded
"test_kallsyms_b" module name.

Fixes: 84b4a51f ("selftests: add new kallsyms selftests")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>