Detect leading spaces in config files

[PVince81]

Because those will cause PHP to output to stdout and will cause
unrelated error messages.

Steps to reproduce:

1. Create a config file "config/app.config.php" and add your apps_paths section (or anything else)
2. In that config file, add spaces in front of <?php
3. Open Nextcloud

Before: internal server error with mysterious errors in the log about ini settings that cannot be set at this stage

After: explicit error about the invalid file, both in occ and web UI

Background: I lost some time with this, believing that an app was broken. In my setup I have a bash script that generates the sidecar config files using cat << and I mistakenly indented the contents of the file

[PVince81]

the tests fail, maybe the bootstrapping thing is generating files like I did... hope it's not a can of worms

[PVince81]

ok, I suspect that maybe something else during the tests is triggering an output and making header_sent() to return true

I'll add a condition to detect testing also

[nickvergessen]

ok, I suspect that maybe something else during the tests is triggering an output and making header_sent() to return true

Could also only check that if header_sent() was false before loading the config files