chore: extract TestEnv into separate crate

`TestEnv` is extracted into its own crate to serve as a framework
for testing other block explorer APIs.

crates/testenv/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "bdk_testenv"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+bitcoincore-rpc = { version = "0.17" }
+bdk_chain = { path = "../chain", version = "0.11", default-features = false }
+electrsd = { version= "0.25.0", features = ["bitcoind_25_0", "esplora_a33e97e1", "legacy"] }
+anyhow = { version = "1" }
+
+[features]
+default = ["std"]
+std = ["bdk_chain/std"]
+serde = ["bdk_chain/serde"]