tests: kernel: sleep: Fix uninitialised variable warning

This commit sets an initial value of 0 for the `elapsed_ms` variable,
which may be used uninitialised when the while loop below does not
execute.

This fixes the "‘elapsed_ms’ may be used uninitialized" warning
generated by the GCC 12.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>