Add unit tests around WEB_CONCURRENCY behavior

WEB_CONCURRENCY should be allowed to be set prior to the `profile.d`
scripts running. This change adds tests to make sure we do not
accidentally clobber preset values.

These tests were extracted from a different change that duplicated
behavior for WEB_CONCURRENCY handling. See the [associated PR][1] for
more information around the discussion.

[1]: #876

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## main
 - Update node version to 14 ([#885](https://github.com/heroku/heroku-buildpack-nodejs/pull/885))
+- Add unit tests around the behavior of `WEB_CONCURRENCY` ([#876](https://github.com/heroku/heroku-buildpack-nodejs/pull/876))
 
 # v182 (2020-01-05)
 - add Node 14.15.3 and 15.5.0 to inventory ([#881](https://github.com/heroku/heroku-buildpack-nodejs/pull/881))

test/unit

@@ -281,6 +281,22 @@ testBuildDataPreviousBuild() {
   assertEquals "$(meta_get test)" ""
 }
 
+testWebConcurrencyDoesNotOverrideManualValue() {
+  WEB_CONCURRENCY=123
+
+  source "$(pwd)"/profile/WEB_CONCURRENCY.sh
+
+  assertEquals 123 "$WEB_CONCURRENCY"
+}
+
+testWebConcurrencySetsAValueWhenUnset() {
+  unset WEB_CONCURRENCY
+
+  source "$(pwd)"/profile/WEB_CONCURRENCY.sh
+
+  assertNotNull "WEB_CONCURRENCY is null" "$WEB_CONCURRENCY"
+}
+
 testWebConcurrencyProfileScript() {
   # this was set when we sourced the WEB_CONCURRENCY.sh file
   unset WEB_MEMORY