chore: Refactor checksum comparisons

- Use a DRY method instead.
- ENV test changed from 2048-bit to 3072-bit to avoid confusion in a future test that should not be mixed up accidentally with 2048-bit elsewhere.
- Custom DH file test comparison changed to match other comparisons for equality against the expected DH param content.
- Related comments revised, additional comment for context added by the test definition.
- Minor white-space adjustments.

test/test_ssl/test_dhparam.yml

@@ -19,7 +19,7 @@ with_default_group:
 with_alternative_group:
   container_name: dh-env
   environment:
-    - DHPARAM_BITS=2048
+    - DHPARAM_BITS=3072
   image: *img-nginxproxy
   volumes: *vols-common
 
@@ -33,7 +33,7 @@ with_invalid_group:
 with_custom_file:
   container_name: dh-file
   image: *img-nginxproxy
-  volumes: 
+  volumes:
     - *docker-sock
     - *nginx-certs
     - ../../dhparam/ffdhe3072.pem:/etc/nginx/dhparam/dhparam.pem:ro
@@ -50,4 +50,4 @@ with_skip_backward:
   environment:
     - DHPARAM_GENERATION=false
   image: *img-nginxproxy
-  volumes: *vols-common
+  volumes: *vols-common