[TRIVIAL] Cleanup the JNI test file

Summary:
Cleanup the NativeSecp256k1Test.java file:

• Move comments on their own lines;
• Add period at the end of sentences;
• Remove extra whitespaces in parenthesis to make the code consistent;
• Add/Remove some newlines where needed;
• Remove commented out debug code;
• Replace tabs with spaces;
• ...

Test Plan:

./contrib/teamcity/build-secp256k1.sh

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3834