std lib: extend test runner capabilities (#8499)

I am implementing a nu plugin, and want to unit test that it works well.

# Basic usage (unchanged)


![image](https://user-images.githubusercontent.com/282320/225895139-f1ea0088-22e1-4778-b27e-c1868af48753.png)

# Select the folder to run tests within (subfolders included)


![image](https://user-images.githubusercontent.com/282320/225894283-4c6ac739-afde-44ef-bf7e-362d5118e83f.png)

# Select module to run tests within


![image](https://user-images.githubusercontent.com/282320/225894438-f39690db-955d-4d66-818f-17a179b00c66.png)

# Select test command to run


![image](https://user-images.githubusercontent.com/282320/225894534-edb15b9c-d3ef-4368-9108-2d220ef75dcf.png)

# Complex usage


![image](https://user-images.githubusercontent.com/282320/225894827-03f2e937-3984-4b33-b206-c155c723feac.png)

---------

Co-authored-by: Mate Farkas <Mate.Farkas@oneidentity.com>

crates/nu-utils/standard_library/tests.nu

@@ -1,20 +1,55 @@
 use std.nu *
 
-def main [] {
-    for test_file in (ls ($env.FILE_PWD | path join "test_*.nu") -f | get name) {
+def collect-modules [
+    path: path,
+    module?: string
+] {
+    let tests_path = ($path | default $env.FILE_PWD)
+    let module_search = ($module | default "test_*")
+    (ls ($tests_path | path join $"**/($module_search).nu") -f | get name)    
+}
+
+def collect-commands [
+    test_file: string,
+    module_name: string,
+    command?: string
+] {
+    let commands = (
+        nu -c $'use ($test_file) *; $nu.scope.commands | select name module_name | to nuon'
+        | from nuon
+        | where module_name == $module_name
+        | where ($it.name | str starts-with "test_")
+        | get name
+    )
+    if $command == null {
+        $commands
+    } else {
+        $commands | where $it == $command
+    }
+}
+
+# Test executor
+#
+# It executes exported "test_*" commands in "test_*" modules
+def main [
+    --path: path, # Path to look for tests. Default: directory of this file.
+    --module: string, # Module to run tests. Default: all test modules found.
+    --command: string, # Test command to run. Default: all test command found in the files.
+    --list, # Do not run any tests, just list them (dry run)
+] {
+    let dry_run = ($list | default false)
+    for test_file in (collect-modules $path $module) {
         let $module_name = ($test_file | path parse).stem
 
-        log info $"Run tests in ($module_name) module"
-        let tests = (
-            nu -c $'use ($test_file) *; $nu.scope.commands | select name module_name | to nuon'
-            | from nuon
-            | where module_name == $module_name
-            | where ($it.name | str starts-with "test_")
-            | get name
-        )
+        log info $"Run tests in ($module_name)"
+        let tests = (collect-commands $test_file $module_name $command)
 
         for test_case in $tests {
             log debug $"Run test ($module_name) ($test_case)"
+            if $dry_run {
+                continue
+            }
+
             nu -c $'use ($test_file) ($test_case); ($test_case)'
         }
     }