These assertions were checking for deletion of the wrong file paths for
the scenario. Although the step is worded "should have been removed"
it is in practice only checking that the file does not exist - and
therefore was passing because the files had never been created.

Updated to the correct names of the files.

Previously, we were running the scenario in the fixture directory itself
(or explicitly copying individual files to a temporary directory). This
risks Behat modifying fixture files during the scenario (potentially
unexpectedly) and therefore affecting subsequent scenarios.

Instead, run every scenario in an isolated temporary working directory,
and simply initialise this from the fixture at the beginning. This
guarantees that every scenario runs in an isolated environment.

It also simplifies our step definitions, as we no longer need to provide
for copying individual files to a temp dir or having separate steps to
assert the content / presence of files in the temp dir.

Placeholders to some paths in features have changed because the paths
are now relative to the %%WORKING_DIR%% placeholder (and are not nested
under the fixture directory path) rather than to the project root.

Now that we have symfony/filesystem in dev, we can use it to simplify
creating directories and files during our own behat execution. This also
provides for more robust handling of unexpected error cases.

Using symfony/filesystem to clean up our temporary test directories also
fixes a bug that the previous ::clearDirectory method was only removing
files within the temporary directories - the directories themselves were
never removed.

The existing `"foo.php" file should contain` step passes the actual file
content through the `getExpectedOutput()` function that is also used
when asserting command output.

This function automagically converts directory separators in paths into
the DIRECTORY_SEPARATOR for the platform, as well as replacing various
other placeholders.

This fails when trying to assert the content of files generated by the
convert-config command with any `removePrefix` paths configured. The
generated files contain paths with the literal `/` character (matching
the content of the YAML) on all platforms.

Instead, assert the exact content of these files, only normalising
newlines.

Again pending review of how we consistently / reliably handle path and
directory separator issues across all our scenarios.

On MacOS and Windows runners, the path returned by sys_get_temp_dir
(and stored in the workingDir variable) is not a real path. Therefore,
it does not match when passed as an option (e.g. for --remove-prefix)
into Behat. We need to convert to a realpath so that it matches the
actual path to the files on disk.