coverage: lower coverage for source/common/io/ (#32150)

Coverage CI is failing because coverage is too low in source/common/io because CI is not executing io_uring code. #32149

Signed-off-by: Ryan Hamilton <rch@google.com>
Signed-off-by: Ryan Northey <ryan@synca.io>

test/per_file_coverage.sh

@@ -3,14 +3,15 @@
 # directory:coverage_percent
 # for existing directories with low coverage.
 declare -a KNOWN_LOW_COVERAGE=(
-"source/common:96.2"
+"source/common:96.1"
 "source/common/api:84.5"
 "source/common/api/posix:81.8"
 "source/common/config:94.8"
 "source/common/crypto:88.1"
 "source/common/event:95.1" # Emulated edge events guards don't report LCOV
 "source/common/filesystem/posix:96.2" # FileReadToEndNotReadable fails in some env; createPath can't test all failure branches.
 "source/common/http/http2:95.2"
+"source/common/io:6.7"
 "source/common/json:93.7"
 "source/common/matcher:94.6"
 "source/common/network:94.4" # Flaky, `activateFileEvents`, `startSecureTransport` and `ioctl`, listener_socket do not always report LCOV