backupccl: test that we actually downloaded all data during online restore

[msbutler]

Release note: none

Epic: none

[cockroach-teamcity]

This change is 

[dt]

I think we could slash the size of this change substantially and still get the utility we want to get -- asserting we actually downloaded -- without as much additional test code that we need to maintain/comprehend to read a test, round trips to the DB that make our tests slower to run and can flake, and interactions with system tables.

All we really care about is "are there any external bytes in the cluster?" after the download phase finishes. We could just put that query in a const and then just exec where we need it right?

[msbutler]

given that the query has been simplified so much, it's now a const that any test can use.

[msbutler]

unrelated test flake

[msbutler]

TFTR!

bors r+

[craig[bot]]

Build failed (retrying...):

• local_roachtest

[craig[bot]]

Build failed:

• local_roachtest

[msbutler]

bors r=dt

[craig[bot]]

Build succeeded:

• macos_arm64_build
• check_generated_code
• lint
• linux_arm64_build
• local_roachtest_fips
• windows_build
• examples_orms
• docker_image_amd64
• unit_tests
• linux_amd64_fips_build
• local_roachtest
• linux_amd64_build
• macos_amd64_build
• acceptance