test: Optimize sha2 even in debug builds

This makes the tests finish on my machine twice as fast. This works
mainly because some tests utilize pbkdf2 to derive a key from a
passphrase.

Cargo.toml

@@ -20,3 +20,4 @@ lto = true
 # Optimize quote even in debug mode. Speeds up proc-macros enough to account
 # for the extra time of optimizing it for a clean build of matrix-sdk-ffi.
 quote = { opt-level = 2 }
+sha2 = { opt-level = 2 }