tests: fp_sharing: Fix definition of PI_NUM_ITERATIONS

The fp_sharing test has an option to control the number of iterations in
the pi calculation, which is used to adjust the duration of the test on
different platforms that may have significantly different execution
frequencies (e.g., qemu_x86 vs. frdm_k64f). The conversion to CMake did
not handle this option correctly and forced the same value for all
platforms.

The test now completes in about 5 minutes on frdm_k64f.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>