REGTESTS: don't rely on the stick-table dump order in the ipmask test

REGTESTS: don't rely on the stick-table dump order in the ipmask test

"show table" dumps the entries bucket by bucket, so their order depends
on CONFIG_HAP_TBL_BUCKETS, which is 1 in builds without threads (e.g.
TARGET=generic) and 8 or more with them. The test hardcoded the order
seen with multiple buckets and failed on single-bucket builds.

Check the header and each of the three entries with a separate request
instead, so the order doesn't matter anymore.