Make test env more robust

I.e., don't use user cache and temp dirs

tests/mod.nu

@@ -6,10 +6,20 @@ use ../nupm
 const TEST_REGISTRY_PATH = ([tests packages registry.nuon] | path join)
 
 
-def with-nupm-home [closure: closure]: nothing -> nothing {
-    let dir = tmp-dir test --ensure
-    with-env { NUPM_HOME: $dir } $closure
-    rm --recursive $dir
+def with-test-env [closure: closure]: nothing -> nothing {
+    let home = tmp-dir nupm_test --ensure
+    let cache = tmp-dir 'nupm_test/cache' --ensure
+    let temp = tmp-dir 'nupm_test/temp' --ensure
+
+    with-env {
+        NUPM_HOME: $home
+        NUPM_CACHE: $cache
+        NUPM_TEMP: $temp
+    } $closure
+
+    rm --recursive $temp
+    rm --recursive $cache
+    rm --recursive $home
 }
 
 # Examples:
@@ -20,7 +30,7 @@ def "assert installed" [path_tokens: list<string>] {
 }
 
 export def install-script [] {
-    with-nupm-home {
+    with-test-env {
         nupm install --path tests/packages/spam_script
 
         assert installed [scripts spam_script.nu]
@@ -29,7 +39,7 @@ export def install-script [] {
 }
 
 export def install-module [] {
-    with-nupm-home {
+    with-test-env {
         nupm install --path tests/packages/spam_module
 
         assert installed [scripts script.nu]
@@ -39,7 +49,7 @@ export def install-module [] {
 }
 
 export def install-custom [] {
-    with-nupm-home {
+    with-test-env {
         nupm install --path tests/packages/spam_custom
 
         assert installed [plugins nu_plugin_test]
@@ -52,19 +62,19 @@ export def install-from-local-registry [] {
         assert ($contents | str contains '0.2.0')
     }
 
-    with-nupm-home {
+    with-test-env {
         $env.NUPM_REGISTRIES = {}
         nupm install --registry $TEST_REGISTRY_PATH spam_script
         check-file
     }
 
-    with-nupm-home {
+    with-test-env {
         $env.NUPM_REGISTRIES = { test: $TEST_REGISTRY_PATH }
         nupm install --registry test spam_script
         check-file
     }
 
-    with-nupm-home {
+    with-test-env {
         $env.NUPM_REGISTRIES = { test: $TEST_REGISTRY_PATH }
         nupm install spam_script
         check-file