Pin serde_test to 1.0.152

This is just a dev dependency so I did not think that hard about it,
just took a stab at using the same version that we in for `serde` and it
works.

Author: tcharding

contrib/test.sh

@@ -28,6 +28,7 @@ if cargo --version | grep "1\.48\.0"; then
     cargo update -p serde_json --precise 1.0.99
     cargo update -p serde --precise 1.0.152
     cargo update -p log --precise 0.4.18
+    cargo update -p serde_test --precise 1.0.152
 fi
 
 # Test bitcoind integration tests if told to (this only works with the stable toolchain)