gnu: thefuck: Upgrade to 3.15 and enable tests.

* gnu/packages/patches/thefuck-test-environ.patch: New patch.
* gnu/packages/admin.scm (thefuck): Upgrade to 3.15.
[source]: Use patch.
[arguments]: Remove '#:tests? #f'.  Move 'check' phase after 'install'.
* gnu/local.mk (dist_patch_DATA): Add patch.