Add testscript tests for PATH setting (#646)

## Summary
Tests that the PATH is built in the desired order. Had to add a `print`
function and `path.order` assertion function. Let me know if you have
any other ideas!

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

Author: ipince

testscripts/assert/assert.test.txt

@@ -0,0 +1,7 @@
+# Tests assert functions (since some are relatively complex)
+
+exec print a:b:c:d:e:fg:h
+path.order 'a' 'c' 'e'
+! path.order 'a' 'b' 'a'
+path.order 'a' 'f'
+! path.order 'a' 'c' 'h' 'h'

testscripts/run/path.test.txt

@@ -19,14 +19,16 @@ exec devbox run echo '$PATH'
 ! stdout '/some//dirty/../clean/path'
 stdout '/some/clean/path'
 
-# Path contains path to installed nix packages
-stdout '-which-'
-
 # Path contains plugin virtual env path
 stdout '.devbox/virtenv/bin'
 
-# Path contains devbox-managed nix profiel path
+# Path contains devbox-managed nix profile path
 stdout '.devbox/nix/profile/default/bin'
 
-# TODO: verify that paths appear in desired order.
+# Path contains path to installed nix packages
+stdout '-which-'
+
+# Verify PATH is set in correct order: virtual env path, nix profile, nix packages, host path.
+path.order '.devbox/virtenv/bin' '.devbox/nix/profile/default/bin' '-which-' 'some/clean/path'
+
 # TODO: verify that bashrc file prepends do not prepend before nix paths.

testscripts/testrunner/assert.go

@@ -13,6 +13,7 @@ import (
 var assertionMap = map[string]func(ts *testscript.TestScript, neg bool, args []string){
 	"env.path.len":  assertPathLength,
 	"json.superset": assertJSONSuperset,
+	"path.order":    assertPathOrder,
 }
 
 // Usage: env.path.len <number>
@@ -69,3 +70,49 @@ func assertJSONSuperset(script *testscript.TestScript, neg bool, args []string)
 		}
 	}
 }
+
+// Usage: path.order 'a' 'b' 'c'
+// Checks that whatever is in stdout, P, is a string in PATH format (i.e. colon-separated strings), and that
+// every one of the arguments ('a', 'b', and 'c') are contained in separate subpaths of P, exactly once, and
+// in order.
+func assertPathOrder(script *testscript.TestScript, neg bool, args []string) {
+	path := script.ReadFile("stdout")
+	subpaths := strings.Split(strings.Replace(path, "\n", "", -1), ":")
+
+	allInOrder := containsInOrder(subpaths, args)
+	if !neg && !allInOrder {
+		script.Fatalf("Did not find all expected in order in subpaths.\n\nSubpaths: %v\nExpected: %v", subpaths, args)
+	}
+	if neg && allInOrder {
+		script.Fatalf("Found all expected in subpaths.\n\nSubpaths: %v\nExpected: %v", subpaths, args)
+	}
+}
+
+func containsInOrder(subpaths []string, expected []string) bool {
+	if len(expected) == 0 {
+		return true // no parts passed in, assertion trivially holds.
+	}
+
+	if len(subpaths) < len(expected) {
+		return false
+	}
+
+	i := 0
+	j := 0
+outer:
+	for j < len(expected) {
+		currentExpected := expected[j]
+		for i < len(subpaths) {
+			if strings.Contains(subpaths[i], currentExpected) {
+				j++
+				i++
+				continue outer // found expected, move on to the next expected
+			} else {
+				i++ // didn't find it, try the next subpath
+			}
+		}
+		return false // ran out of subpaths, but not out of expected, so we fail.
+	}
+
+	return true // if we're here, we found everything
+}

testscripts/testrunner/testrunner.go

@@ -2,8 +2,10 @@ package testrunner
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/bmatcuk/doublestar/v4"
@@ -18,6 +20,10 @@ func Main(m *testing.M) int {
 			// Call the devbox CLI directly:
 			return boxcli.Execute(context.Background(), os.Args[1:])
 		},
+		"print": func() int { // Not 'echo' because we don't expand variables
+			fmt.Println(strings.Join(os.Args[1:], " "))
+			return 0
+		},
 	}
 	return testscript.RunMain(m, commands)
 }