Force C.UTF-8 or C locale for tests

Force C.UTF-8 or C locale for tests

Otherwise several tests fail, for example in this build done in a French
locale by Debian's reproducible builds initiative, to check whether
the resulting binaries are identical to what was produced in an
English locale:

<https://tests.reproducible-builds.org/debian/logs/unstable/amd64/ostree_2016.11-1.build2.log.gz>

(test-basic)
# error: Cannot write to repository: Permission non accordée
...
File 'error-message' doesn't match regexp 'Permission denied'

(test-help)
# Utilisation :
#   ostree [OPTION...] COMMAND
...
File 'out' doesn't match regexp '[Uu]sage'

(test-pull-metalink)
# error: Erreur à la ligne 1, caractère 1 : Le document doit commencer avec un élément (par ex. <book>)
...
File 'err.txt' doesn't match regexp 'Document must begin with an element'

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #558
Approved by: cgwalters