[secp256k1] fix java secp256k1 test

Summary: fix java secp256k1 test

Test Plan:
to run the java secp256k1 tests (including this one):

$ cd src/secp256k1
$ ./autogen.sh
$ ./configure --enable-jni --enable-experimental --enable-module-ecdh
$ make check-java

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2685