[dist] Add a environment configuration for our custom migration env

This commit links config/environments/migration.rb to config/environments/test.rb.
The migration env is used by a so called migration test we run in our
rpm package builds.

This should solve following error / warning that appear during our
package builds:

[  378s] Created database 'api_25'
[  383s] config.eager_load is set to nil. Please update your config/environments/*.rb files accordingly:
[  383s]
[  383s]   * development - set it to false
[  383s]   * test - set it to false (unless you use a tool that preloads your test environment)
[  383s]   * production - set it to true
[  383s]

src/api/config/environments/migration.rb

@@ -0,0 +1 @@
+test.rb