test: lib: mem_alloc: enable user mode in newlibcnano test

libc.newlibcnano test-case shall run with user mode
enabled, similarly to the remainder of the test-cases
in the tests/lib/mem_alloc test-suite.

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