Fix realPathMap in test harness: Must be used in directoryExists

[ghost]

Test originally added by #8486
Was broken during a merge commit #9169

[vladima]

nit: can we instead have a function

function mapHasFileInDirectory(path: Path, map: FileMap<any>): boolean {
    if (!map) {
        return false;
    }
    let exists = false;
    map.forEachValue(fileName => {
        if (!exists && startsWith(fileName, path) && fileName[path.length] === "/") {
            exists = true;
        }
    });
    return exists;
}

then original code can be rewritten as

const exists  = mapHasFileInDirectory(path, fileMap) || mapHasFileInDirectory(path, realPathMap);