build: install diffutils in test container image

Depending on the local changes, running 'make containerized-test' fails
with an error like:

    level=error msg="Running error: gofmt: error computing diff: exec: \"diff\": executable file not found in $PATH"

Installing the diffutils package makes sure 'go fmt' finds the
executable.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

scripts/Dockerfile.test

@@ -31,6 +31,7 @@ RUN source /build.env \
 	ShellCheck \
 	yamllint \
 	npm \
+	diffutils \
       python3-prettytable \
       pylint \
     && dnf -y update \