Add --plugins flag to nu-std/testing.nu (#15552)

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# Description
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In this PR I added the flag `--plugins` to the `testing.nu` file inside
of `crates/nu-std`. This allows running tests with active plugins. While
I did not use it here in this repo, it allows testing in
[nushell/plugin-examples](https://github.com/nushell/plugin-examples)
with plugins.

# User-Facing Changes
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helps us keep track of breaking changes. -->
None, just the additional flag.

# Tests + Formatting
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- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

(nothing broke \o/)

# After Submitting
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crates/nu-std/testing.nu

@@ -132,6 +132,7 @@ def show-pretty-test [indent: int = 4] {
 # * Modified file is removed
 def run-test [
     test: record
+    plugins: list<string>
 ] {
     let test_file_name = (random chars --length 10)
     let test_function_name = (random chars --length 10)
@@ -155,8 +156,9 @@ export def ($test_function_name) [] {
     | str join (char lsep)
     | save $rendered_module_path
 
+    let plugins = $plugins | to json -r
     let result = (
-        ^$nu.current-exe --no-config-file -c $"use ($rendered_module_path) *; ($test_function_name)|to nuon"
+        ^$nu.current-exe --no-config-file --plugins $plugins -c $"use ($rendered_module_path) *; ($test_function_name)|to nuon"
         | complete
     )
 
@@ -175,6 +177,7 @@ export def ($test_function_name) [] {
 def run-tests-for-module [
     module: record<file: path name: string before-each: string after-each: string before-all: string after-all: string test: list test-skip: list>
     threads: int
+    plugins: list<string>
 ]: nothing -> table<file: path, name: string, test: string, result: string> {
     let global_context = if not ($module.before-all|is-empty) {
             log info $"Running before-all for module ($module.name)"
@@ -183,7 +186,7 @@ def run-tests-for-module [
                 before-each: 'let context = {}',
                 after-each: '',
                 test: $module.before-all
-            }
+            } $plugins
             | if $in.exit_code == 0 {
                 $in.stdout
             } else {
@@ -233,7 +236,7 @@ def run-tests-for-module [
             log debug $"Global context is ($global_context)"
 
             $test|insert result {|x|
-                run-test $test
+                run-test $test $plugins
                 | if $in.exit_code == 0 {
                     'pass'
                 } else {
@@ -253,7 +256,7 @@ def run-tests-for-module [
                 before-each: $"let context = ($global_context)",
                 after-each: '',
                 test: $module.after-all
-        }
+        } $plugins
     }
     return $tests
 }
@@ -278,6 +281,7 @@ export def run-tests [
     --exclude-module: string,             # Pattern to use to exclude test modules. Default: No modules are excluded
     --list,                               # list the selected tests without running them.
     --threads: int@"nu-complete threads", # Amount of threads to use for parallel execution. Default: All threads are utilized
+    --plugins: list<string>,              # Plugins to load while running tests.
 ] {
     let available_threads = (sys cpu | length)
 
@@ -356,7 +360,7 @@ export def run-tests [
     let results = (
         $modules
         | par-each  --threads $threads {|module|
-            run-tests-for-module $module $threads
+            run-tests-for-module $module $threads ($plugins | default [])
         }
         | flatten
     )