tests: minitest + rack-test suite, bin/test with ephemeral Postgres

Gemfile.lock

@@ -51,6 +51,7 @@ GEM
       net-smtp
     matrix (0.4.3)
     mini_mime (1.1.5)
+    minitest (5.27.0)
     multi_json (1.21.1)
     mustermann (3.1.1)
     net-imap (0.6.4)
@@ -87,8 +88,11 @@ GEM
     rack-session (2.1.2)
       base64 (>= 0.1.0)
       rack (>= 3.0.0)
+    rack-test (2.2.0)
+      rack (>= 1.3)
     rackup (2.3.1)
       rack (>= 3)
+    rake (13.4.2)
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
@@ -138,6 +142,7 @@ DEPENDENCIES
   dotenv (~> 3.1)
   erubi (~> 1.13)
   mail (~> 2.8)
+  minitest (~> 5.25)
   pg (~> 1.5)
   prawn (~> 2.5)
   prawn-table (~> 0.2)
@@ -145,7 +150,9 @@ DEPENDENCIES
   rack (~> 3.2)
   rack-protection (~> 4.2)
   rack-session (~> 2.1)
+  rack-test (~> 2.2)
   rackup (~> 2.3)
+  rake (~> 13.2)
   rerun (~> 0.14)
   rexml (~> 3.4)
   rqrcode (~> 2.2)