* Add `GlobalCommandTest.php`
- `testGlobal` to check `COMPOSER_HOME` is followed correctly + check
`COMPOSER` is unset.
- `testNotCreateHome` to test handling invalid `COMPOSER_HOME`.
* Add error string for non obvious test case
* Clean up env vars and minor code style changes
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Co-authored-by: Jordi Boggiano <j.boggiano@seld.be>
