tests: lib: sprintf: filter-out MCIMX7_M4 platform

Apparently the tests/lib/sprintf test requires more than 34kB
of code size, when building in MCIMX7_M4-based platforms. Such
platforms, however, only have 32kB of code memory, therefore,
we exclude them from this test.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>