test-runner: add configurable threading (#9628)

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# Description
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Max amount of threads used by the test runner can now be configured via
the `--threads` flag

# User-Facing Changes
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helps us keep track of breaking changes. -->

# Tests + Formatting
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crates/nu-std/std/testing.nu

@@ -1,5 +1,8 @@
 use log.nu
 
+def "nu-complete threads" [] {
+    seq 1 (sys|get cpu|length)
+}
 
 # Here we store the map of annotations internal names and the annotation actually used during test creation
 # The reason we do that is to allow annotations to be easily renamed without modifying rest of the code
@@ -202,6 +205,7 @@ export def ($test_function_name) [] {
 # * result - test execution result
 def run-tests-for-module [
     module: record<file: path name: string before-each: string after-each: string before-all: string after-all: string test: list test-skip: list>
+    threads: int
 ] {
     let global_context = if not ($module.before-all|is-empty) {
             log info $"Running before-all for module ($module.name)"
@@ -255,7 +259,7 @@ def run-tests-for-module [
                 ''
             }
         }
-        | par-each {|test|
+        | par-each  --threads $threads {|test|
             log info $"Running ($test.test) in module ($module.name)"
             log debug $"Global context is ($global_context)"
 
@@ -304,8 +308,22 @@ export def run-tests [
     --exclude: string,                    # Pattern to use to exclude tests. Default: no tests are excluded
     --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
 ] {
 
+    let available_threads = (sys | get cpu | length)
+
+    # Can't use pattern matching here due to https://github.com/nushell/nushell/issues/9198
+    let threads = (if $threads == null {
+        $available_threads
+    } else if $threads < 1 {
+        1
+    } else if $threads <= $available_threads {
+        $threads
+    } else {
+        $available_threads
+    })
+
     let module_search_pattern = ('**' | path join ({
         stem: ($module | default "*")
         extension: nu
@@ -336,7 +354,7 @@ export def run-tests [
 
     let modules = (
         ls ($path | path join $module_search_pattern)
-        | par-each {|row| {file: $row.name name: ($row.name | path parse | get stem)}}
+        | par-each --threads $threads {|row| {file: $row.name name: ($row.name | path parse | get stem)}}
         | insert commands {|module|
             get-annotated $module.file
         }
@@ -366,8 +384,8 @@ export def run-tests [
 
     let results = (
         $modules
-        | par-each {|module|
-            run-tests-for-module $module
+        | par-each  --threads $threads {|module|
+            run-tests-for-module $module $threads
         }
         | flatten
     )
@@ -375,7 +393,7 @@ export def run-tests [
         let text = ([
             $"(ansi purple)some tests did not pass (char lparen)see complete errors below(char rparen):(ansi reset)"
             ""
-            ($results | par-each {|test| ($test | show-pretty-test 4)} | str join "\n")
+            ($results | par-each   --threads $threads {|test| ($test | show-pretty-test 4)} | str join "\n")
             ""
         ] | str join "\n")