parameterize class caching in test

closes FOO-2539
flag=none

TEST PLAN:
  1) specs pass

Change-Id: Ibd4030767a8f015b3c197a568d7bb26a91dca92d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/277273
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>

Jenkinsfile

@@ -303,6 +303,16 @@ pipeline {
     CANVAS_LMS_REFSPEC = getCanvasLmsRefspec()
     DOCKER_WORKDIR = getDockerWorkDir()
     LOCAL_WORKDIR = getLocalWorkDir()
+
+    // TEST_CACHE_CLASSES is consumed by config/environments/test.rb
+    // to decide whether to allow class reloading or not.
+    // in local development we usually want this unset or set to '0' because
+    // we want spring to be able to reload classes between
+    // spec runs, but this is expensive when running all the
+    // specs for the build.  EVERYWHERE in the build we want
+    // to be able to cache classes because they don't change while the build
+    // is running and should never be reloaded.
+    TEST_CACHE_CLASSES = '1'
   }
 
   stages {

config/environments/test.rb

@@ -24,7 +24,12 @@ environment_configuration(defined?(config) && config) do |config|
   # test suite.  You never need to work with it otherwise.  Remember that
   # your test database is "scratch space" for the test suite and is wiped
   # and recreated between test runs.  Don't rely on the data there!
-  config.cache_classes = true
+  # In local development we usually want this unset or set to '0' because
+  # we want spring to be able to reload classes between
+  # spec runs, but this is expensive when running all the
+  # specs for the build, so this ENV var can be provided by
+  # any harness to turn class caching on for performance.
+  config.cache_classes = (ENV.fetch("TEST_CACHE_CLASSES", "0") == "1")
 
   # Show formatted error reports and disable caching
   config.consider_all_requests_local = false

docker-compose.new-jenkins.yml

@@ -19,6 +19,7 @@ services:
       FORCE_FAILURE:
       CANVAS_RAILS6_0:
       CANVAS_ZEITWERK:
+      TEST_CACHE_CLASSES:
 
       # parallel_tests
       CI_NODE_TOTAL: