Improve assertions in `IntegrationTests2` `testConfigOptionComments`

Improve assertions in `IntegrationTests2` `testConfigOptionComments`

Before, failing asserts in this test would look like this:

```
Writing default configuration to
/Users/wiggles/cabal/cabal-install/tests/IntegrationTests2/config/cabal-config/config
Downloading the latest package list from hackage.haskell.org
Package list of hackage.haskell.org has been updated.
The index-state is set to 2024-09-04T15:12:07Z.
Integration tests (internal)
  Flag tests
    Test Config options for commented options: FAIL
      tests/IntegrationTests2.hs:2015:
      expected: "  urll"
       but got: "  url"
```

The output includes a message that a configuration file has been
written, but that's not the configuration file under test! The actual
configuration file being tested is
`cabal-install/tests/IntegrationTests2/config/default-config`, which
shows up nowhere in the output.

Now, the messages include the setting name being searched for and the
path of the relevant configuration file:

```
Downloading the latest package list from hackage.haskell.org
Package list of hackage.haskell.org has been updated.
The index-state is set to 2024-09-04T22:53:05Z.
Integration tests (internal)
  Flag tests
    Test Config options for commented options: FAIL
      tests/IntegrationTests2.hs:2015:
      Did not find expected line for setting "url" in configuration file /Users/wiggles/cabal/cabal-install/tests/IntegrationTests2/config/default-config
      expected: "  urll"
       but got: "  url"
```

The `Writing default configuration to...` message with the misleading
path has also been silenced.