CI: Skip tests on Ubuntu Xenial

Due meson error:
WARNING: You are using 'ANSI_X3.4-1968' which is not a a Unicode-compatible locale.
WARNING: You might see errors if you use UTF-8 strings as filenames, as strings, or as file contents.
WARNING: Please switch to a UTF-8 locale for your platform.

Link: mesonbuild/meson#2268

Signed-off-by: Petr Vorel <pvorel@suse.cz>

.github/workflows/ci.yml

@@ -47,6 +47,8 @@ jobs:
             env:
               CC: clang
               EXTRA_BUILD_OPTS: "-DUSE_GETTEXT=false"
+              # meson error: not using Unicode-compatible locale (ANSI_X3.4-1968)
+              SKIP_TESTS: true
 
           - container: "ubuntu:groovy"
             env:
@@ -108,4 +110,4 @@ jobs:
       run: ./build.sh install
 
     - name: Run tests
-      run: ./build.sh test
+      run: if [ ! "$SKIP_TESTS" ]; then ./build.sh test; else echo "tests skipped"; fi