docker: gracefully skip check_qemu

Not all our images are able to run the tests. Rather than use features
we can just check for the existence and run-ability of gtester. If the
image has been setup for binfmt_misc it will be able to run anyway.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/common.rc

@@ -48,7 +48,13 @@ check_qemu()
     else
         INVOCATION="$@"
     fi
-    make $MAKEFLAGS $INVOCATION
+
+    if command -v gtester > /dev/null 2>&1 && \
+           gtester --version > /dev/null 2>&1; then
+        make $MAKEFLAGS $INVOCATION
+    else
+        echo "No working gtester, skipping make $INVOCATION"
+    fi
 }
 
 test_fail()