CI: Pin dependencies for MSRV build

Pin three dependencies (direct and transient) that break MSRV:

- serde_json
- serde
- wasm-bindgen-test

contrib/test.sh

@@ -13,6 +13,12 @@ if cargo --version | grep nightly; then
     NIGHTLY=true
 fi
 
+# Pin dependencies as required if we are using MSRV toolchain.
+if cargo --version | grep "1\.48"; then
+    cargo update -p wasm-bindgen-test --precise 0.3.34
+    cargo update -p serde --precise 1.0.156
+fi
+
 # Test if panic in C code aborts the process (either with a real panic or with SIGILL)
 cargo test -- --ignored --exact 'tests::test_panic_raw_ctx_should_terminate_abnormally' 2>&1 | tee /dev/stderr | grep "SIGILL\\|panicked at '\[libsecp256k1\]"