[10.x] Fix Process::fake() never matching multi-line commands

[SjorsO]

It is currently impossible to use patterns with Process::fake() to match a multi-line command. For example:

$result = Process::run(<<<BASH
git clone --depth 1 \
      --single-branch \
      --branch main \
      git://some-url .
BASH);

Process::preventStrayProcesses();

Process::fake(['*' => 'faked output']);

The test above will always throw an "Attempted process without a matching fake" exception. This is because the pattern is matched using Str::is(), and Str::is() always returns false for multi-line strings (the regex it uses doesn't have the m option).

This PR is only a partial fix, using any pattern that isn't * (like git clone* for example) still won't work if the command has multiple lines.

I'm not sure what the best approach for a proper fix is. I assume modifying Str::is() isn't an option.

I know it isn't ideal that this PR isn't a proper fix, but this change at least makes it possible to test multi-line commands. I'm currently resorting to monkey patching the PendingProcess file in the vendor directory. I could re-write my commands to not be multi-line, but some of them are pretty long so I'd rather not do that.

Related issue: #50158