Add testscript tests for config env vars (#643)

## Summary
TSIA

## How was it tested?
```
go test ./testscripts/testscripts_test.go
```

Author: ipince

testscripts/run/env.test.txt

@@ -1,4 +1,4 @@
-exec devbox init
+# Tests related to setting the environment for devbox run.
 
 # Random vars should not leak into the run environment
 env FOO=bar
@@ -24,4 +24,24 @@ env DEVBOX_FOO=baz
 exec devbox run echo '$DEVBOX_FOO'
 stdout 'baz'
 
-# TODO: test order/overwriting of variables
+# Vars defined in devbox.json are passed through
+env DEVBOX_FEATURE_ENV_CONFIG=1
+exec devbox run echo '$CONFIG_VAR1'
+stdout 'abc'
+
+# Vars defined in devbox.json that reference another variable are set
+env DEVBOX_FEATURE_ENV_CONFIG=1
+env DEVBOX_FOO=baz
+exec devbox run echo '$CONFIG_VAR2'
+stdout 'baz'
+
+# TODO: test order/overwriting of variables
+
+-- devbox.json --
+{
+  "packages": [],
+  "env": {
+    "CONFIG_VAR1": "abc",
+    "CONFIG_VAR2": "$DEVBOX_FOO"
+  }
+}