tests: Fix LC_ALL for systems which use .utf8 suffixes

libtest-core.sh tries to clear the locale to a UTF-8 supporting C
locale, either by setting it to C.UTF-8 (preferred) or just C.

Some systems, like Fedora 26, use the locale name C.utf8, rather than
C.UTF-8. Support that too.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1347
Approved by: cgwalters

tests/libtest-core.sh

@@ -37,6 +37,8 @@ assert_not_reached () {
 # (https://sourceware.org/glibc/wiki/Proposals/C.UTF-8)
 if locale -a | grep C.UTF-8 >/dev/null; then
     export LC_ALL=C.UTF-8
+elif locale -a | grep C.utf8 >/dev/null; then
+    export LC_ALL=C.utf8
 else
     export LC_ALL=C
 fi