The previous test would pass no matter what the `ignoreString` was. I
don't know enough Bash/Grep to explain it, but it was easy to test by
just changing the pattern to have `WRONG` instead of `val1`, and that
would still pass the `assertFileContains` test.
I've hopefully fixed it now, by using `lib.escapeShellArg` before
passing it to `assertFileContains`, and I've also added a negative test
case, to ensure that a bad ignore string is _not_ found in the merge
script.
