tests: lib: heap: exclude qemu_riscv32 platform

This CPU-bound test on qemu_riscv32 platform is very slow when
QEMU icount mode enabled, taking upwards of several minutes.
There's little value here, this is a unit test of library code
and we have coverage of the RISC-V 32 bit arch via hifive1.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>