fix: test: fix random failure on synthrecord system test

One of the synthrecord system tests uses a test function to generate an expected name based on some randomly generated IPv6 (using Hypothesis). Turns out the test function generating the name didn't handle the case where the label which encodes the IPv6 could have a leading or trailing '-' character. (The plugin needs to add a leading or trailing 0 so as not to break IDN compatibility.)

Merge branch 'colin/fix-synthrecord-v6test' into 'main'

See merge request isc-projects/bind9!11073