Fix and test sr25519 signing in nostd (#1872)

* Fix and test sr25519 signing in nostd

* Remove sr25519 signing test on nostd for thumbabi target

* Don't use sr25519 feature in nostd tests

* Fix nits, remove WASM deps from nostd test, improve comments

* Change copypasted comment

* fmt

* Update CI to account for signer tests

signer/tests/no-std/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "nostd-tests"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dev-dependencies]
+
+# This crate is not a part of the workspace, to ensure that no features
+# are enabled for it at the workspace level; which conflict with this test.
+subxt-signer = { path = "../../", default-features = false, features = [
+    "sr25519",
+    "ecdsa",
+    "unstable-eth",
+] }
+
+# this shouldn't be needed, it's in workspace.exclude, but still
+# I get the complaint unless I add it...
+[workspace]