[SECP256K1] Check if variable=yes instead of if var is set in travis
Summary:
Right now travis.sh checks if VALGRIND and CTIMETEST are set or not, but in travis.yml we explictly set them to yes so I think it's more intuitive to check if they're equal to yes such that setting CTIMETEST=no(or anything other than "yes") will disable the test. pointed out by @real-or-random here: #696 (comment)
Backport of secp256k1 PR782.
Includes the missing CTIMETEST variable for s390x.
Depends on D7630.
Test Plan: Run the Travis build.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7631