remove libc and add small readme with test instructions

testing/no-std-tests/Cargo.lock

@@ -361,7 +361,6 @@ checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 name = "subxt-core-no-std-tests"
 version = "0.0.0"
 dependencies = [
- "libc",
  "libc_alloc",
  "parity-scale-codec",
  "subxt-metadata",

testing/no-std-tests/Cargo.toml

@@ -8,7 +8,6 @@ resolver = "2"
 [dependencies]
 subxt-metadata = { path = "../../metadata", default-features = false }
 codec = { package = "parity-scale-codec", version = "3.6.9", default-features = false, features = ["derive"] }
-libc = { version = "0.2", default-features = false }
 libc_alloc = { version = "1.0.6" }
 
 [profile.dev]

testing/no-std-tests/README.md

@@ -0,0 +1,7 @@
+# No-Std Testing Crate
+
+To test the no-std compatibility of various subxt-* crates, please run:
+
+```bash
+cargo build --target thumbv7em-none-eabi
+```