Exclude test files from crate (#1187)

### Why

Make the crate smaller (context -
stellar/rs-soroban-env#1294 (comment))

Author: sisuresh

soroban-sdk/Cargo.toml

@@ -10,6 +10,8 @@ version.workspace = true
 edition = "2021"
 rust-version = "1.74"
 
+exclude = ["test_snapshots/", "src/tests/"]
+
 [lib]
 doctest = false