Add tests for env var application order (#645)

## Summary
Also tests the `PWD` special logic

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

Author: ipince

testscripts/run/env.test.txt

@@ -35,13 +35,29 @@ env DEVBOX_FOO=baz
 exec devbox run echo '$CONFIG_VAR2'
 stdout 'baz'
 
-# TODO: test order/overwriting of variables
+# Vars in devbox that refer to $PWD should get the project dir
+env PWD=/test-pwd
+exec devbox run echo '$CONFIG_VAR3'
+! stdout '/test-pwd'
+
+# Variables are applied in order: nix vars, DEVBOX_*, leaked, leakedForShell, fixed/hard-coded vars,
+# plugin vars, and config vars. It really only makes sense to test for plugin and config vars order.
+# Note that the nginx plugin defines NGINX_CONFDIR, NGINX_PATH_PREFIX, and NGINX_TMPDIR.
+env NGINX_TMPDIR="to-be-overwritten-by-plugin"
+exec devbox run echo '$NGINX_TMPDIR'
+! stdout 'to-be-overwritten-by-plugin'
+stdout '.devbox/virtenv/nginx/temp'
+
+exec devbox run echo '$NGINX_CONFDIR'
+stdout 'devbox-json-override'
 
 -- devbox.json --
 {
-  "packages": [],
+  "packages": ["nginx"],
   "env": {
     "CONFIG_VAR1": "abc",
-    "CONFIG_VAR2": "$DEVBOX_FOO"
+    "CONFIG_VAR2": "$DEVBOX_FOO",
+    "CONFIG_VAR3": "${PWD}",
+    "NGINX_CONFDIR": "devbox-json-override"
   }
 }