Use nu-cmd-lang default context for plugin tests (#12434)

# Description
@ayax79 added `nu-cmd-lang` as a dep for `nu-plugin-test-support` in
order to get access to `let`. Since we have the dep anyway now, we might
as well just add all of the lang commands - there aren't very many of
them and it would be less confusing than only `let` working.

# User-Facing Changes
- Can use some more core nu language features in plugin tests, like
loops and `do`

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

# After Submitting
- [ ] Might need to change something about the plugin testing section of
the book, since I think it says something about there only being the
plugin command itself available

crates/nu-plugin-test-support/src/plugin_test.rs

@@ -1,7 +1,7 @@
 use std::{cmp::Ordering, convert::Infallible, sync::Arc};
 
 use nu_ansi_term::Style;
-use nu_cmd_lang::Let;
+use nu_cmd_lang::create_default_context;
 use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_plugin::{Plugin, PluginCommand, PluginCustomValue, PluginSource};
@@ -37,9 +37,8 @@ impl PluginTest {
         name: &str,
         plugin: Arc<impl Plugin + Send + 'static>,
     ) -> Result<PluginTest, ShellError> {
-        let mut engine_state = EngineState::new();
+        let mut engine_state = create_default_context();
         let mut working_set = StateWorkingSet::new(&engine_state);
-        working_set.add_decl(Box::new(Let));
 
         let reg_plugin = fake_register(&mut working_set, name, plugin)?;
         let source = Arc::new(PluginSource::new(reg_plugin));