test: reduce dump count in vinyl-luatest/select_consistency_test

The test expects at least 10 dumps to be created in 60 seconds. It
usually works but sometimes, when the host is heavy loaded, Vinyl
doesn't produce enough dumps in time and fails the test. On CI the test
usually fails with 7-9 dumps. To avoid flaky failures, let's reduce the
expected dump count down to 5.

Closes #10752

NO_DOC=test fix
NO_CHANGELOG=test fix

(cherry picked from commit 5325abd3441ecb4b589799c32ec181d88724b8a8)