Test cstring failure for gcc targets

The test was ported from mbed, where floats are enabled for libraries, which is not a case for our current gcc targets, thus this test fail for gcc, but not for armcc. We should remove floats testing as it's not true for all targets (issue tracking under mbed-target-gcc: https://github.com/ARMmbed/target-mbed-gcc/issues/17)

File reference: https://github.com/ARMmbed/mbed-drivers/blob/master/test/cstring/main.cpp#L63

Open test/cstring/main.cpp at the referenced line and inspect the float-related assertions ported from mbed. Remove the float testing that is not valid for the current gcc targets, then run the cstring test for a gcc target and confirm the test no longer fails while preserving the valid checks.

Written by the indexing model from the issue text.