Test full feature separately in no_std on CI (#424, #420)

## Synopsis

`cargo test --no-default-features --features=full` fails imposing
problems on other CIs.

## Solution

- Consider testing `full` Cargo feature in `test-features` CI job to
handle such regressions.
- Update the relevant tests to run under `no_std` too.

ci/test_all_features.sh

@@ -7,6 +7,6 @@ fi
 
 set -euxo pipefail
 
-for feature in $(tomljson Cargo.toml | jq --raw-output '.features | keys[]' | grep -v 'default\|std\|full\|testing-helpers'); do
+for feature in $(tomljson Cargo.toml | jq --raw-output '.features | keys[]' | grep -v 'default\|std\|testing-helpers'); do
     RUSTFLAGS='-D warnings' cargo +nightly test -p derive_more --tests --no-default-features --features "$feature$std,testing-helpers"
 done

tests/lib.rs

@@ -1,5 +1,12 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
+#[cfg(not(feature = "std"))]
+#[macro_use]
+extern crate alloc;
+
+#[cfg(not(feature = "std"))]
+use alloc::{boxed::Box, string::ToString as _, vec::Vec};
+
 use derive_more::{
     Add, AddAssign, Binary, BitAnd, BitOr, BitXor, Constructor, Deref, DerefMut,
     Display, Div, From, FromStr, Index, IndexMut, Into, IntoIterator, Mul, MulAssign,