tests: net: Fix CoAP coverity issues

Fix unintended sign extension (SIGN_EXTENSION).

The result of "coap_block_size_to_bytes() * (iter - 1)"
extending the sign type.

Issue is the conversion of iter from u8_t to int because it's
an unsigned operand in an binary subtraction with 1 which is
a signed operand with higher conversion rank (int).

Fixes #20880
Fixes #20881
Fixes #20882
Fixes #20883

Coverity CID :205780
Coverity CID :205786
Coverity CID :205806
Coverity CID :205808

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>