[BUGFIX] t3lib_utility_mailTest fails

The tested method always adds a From address to the mail header. This
is not reflected in the test.
Additionally clean up the test header a bit (backup globals).

Resolves: #49361
Releases: 4.5
Change-Id: I33d85916980eea34331d7d87682de9f71119af25
Reviewed-on: https://review.typo3.org/21617
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn