gitlab-ci: Test librem5-devkit-flash-image download

We can't test the actual flashing but that's most of what the script
does atm.

This will make us spot things like fixed in !1 early.

.gitlab-ci.yml

+variables:
+  FLASHDEPS: python3
+             python3-coloredlogs
+             python3-jenkins
+             python3-requests
+             python3-tqdm
+             python3-yaml
 
 .tags: &tags
   tags:
@@ -31,3 +38,16 @@ build:deb:
   artifacts:
     paths:
       - '*.deb'
+
+test:download:
+  <<: *tags
+  stage: test
+  before_script:
+    - export DEBIAN_FRONTEND=noninteractive
+    - apt-get -y update
+    - apt-get -y install $FLASHDEPS
+  script:
+    - scripts/librem5-devkit-flash-image --skip-flash --skip-cleanup --dir=download/
+    - ls -l download/devkit.img download/flash_devkit.lst  download/u-boot-devkit-recovery.imx
+    - rm -rf download
+