Check for proper file size output in dnstap test

Check for proper file size output in dnstap test

Previously, the first check silently failed, as 450 is apparently (in
the CI setup) the minimum output size for the dnstap output, rather than
470 which the test was expecting. Effectively, the check served as a 5
second sleep rather than waiting for the proper file size.

Additionally, check the expected file sizes and fail if expectations
aren't met.

(manually picked from commit 5f809e50b6a8441592fdf6d0fd8aa69668a25439)

On main, the minimum file size seems to 454 bytes, while on EL7 in our
CI setup for the 9.18 branch, it appears to be 450 instead.