[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 [[bitcoin-core/secp256k1#782 | 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

src/secp256k1/.travis.yml

@@ -68,10 +68,10 @@ env:
     - BIGNUM=no
     - BIGNUM=no       ENDOMORPHISM=yes  RECOVERY=yes  EXPERIMENTAL=yes  MULTISET=yes
     - BIGNUM=no       STATICPRECOMPUTATION=no
-    - AUTOTOOLS_TARGET=distcheck       CMAKE_TARGET=install    CTIMETEST= BENCH=
+    - AUTOTOOLS_TARGET=distcheck       CMAKE_TARGET=install    CTIMETEST=no BENCH=no
     - AUTOTOOLS_EXTRA_FLAGS=CPPFLAGS=-DDETERMINISTIC CMAKE_EXTRA_FLAGS=-DCMAKE_C_FLAGS=-DDETERMINISTIC
-    - AUTOTOOLS_EXTRA_FLAGS=CFLAGS=-O0               CMAKE_EXTRA_FLAGS=-DCMAKE_BUILD_TYPE=Debug CTIMETEST=
-    - AUTOTOOLS_TARGET=check-java      CMAKE_TARGET=check-secp256k1-java JNI=yes ECDH=yes EXPERIMENTAL=yes CTIMETEST= BENCH=
+    - AUTOTOOLS_EXTRA_FLAGS=CFLAGS=-O0               CMAKE_EXTRA_FLAGS=-DCMAKE_BUILD_TYPE=Debug CTIMETEST=no
+    - AUTOTOOLS_TARGET=check-java      CMAKE_TARGET=check-secp256k1-java JNI=yes ECDH=yes EXPERIMENTAL=yes CTIMETEST=no BENCH=no
     - ECMULTGENPRECISION=2
     - ECMULTGENPRECISION=8
     - VALGRIND=yes
@@ -102,7 +102,7 @@ jobs:
     # S390x build (big endian system)
     - dist: focal
       compiler: gcc
-      env: HOST=s390x-unknown-linux-gnu OPENSSL_TESTS=no ECDH=yes RECOVERY=yes EXPERIMENTAL=yes MULTISET=yes CTIMETEST=
+      env: HOST=s390x-unknown-linux-gnu OPENSSL_TESTS=no ECDH=yes RECOVERY=yes EXPERIMENTAL=yes MULTISET=yes CTIMETEST=no
       arch: s390x
       addons:
         apt:

src/secp256k1/travis/build_autotools.sh

@@ -61,16 +61,16 @@ trap 'print_logs' ERR
 
 make -j2 $AUTOTOOLS_TARGET
 
-if [ -n "$VALGRIND" ]; then
+if [ "$VALGRIND" = "yes" ]; then
   # the `--error-exitcode` is required to make the test fail if valgrind found
   # errors, otherwise it'll return 0
   # (http://valgrind.org/docs/manual/manual-core.html)
   valgrind --error-exitcode=42 ./tests 16
   valgrind --error-exitcode=42 ./exhaustive_tests
 fi
 
-if [ -n "$BENCH" ]; then
-  if [ -n "$VALGRIND" ]; then
+if [ "$BENCH" = "yes" ]; then
+  if [ "$VALGRIND" = "yes" ]; then
     # Using the local `libtool` because on macOS the system's libtool has
     # nothing to do with GNU libtool
     EXEC='./libtool --mode=execute valgrind --error-exitcode=42';
@@ -91,7 +91,7 @@ if [ -n "$BENCH" ]; then
     $EXEC ./bench_multiset >> bench.log 2>&1
   fi
 fi
-if [ -n "$CTIMETEST" ]; then
+if [ "$CTIMETEST" = "yes" ]; then
   ./libtool --mode=execute valgrind --error-exitcode=42 ./valgrind_ctime_test > valgrind_ctime_test.log 2>&1
 fi
 

src/secp256k1/travis/build_cmake.sh

@@ -49,7 +49,7 @@ ${CMAKE_COMMAND} -GNinja .. \
 
 ninja $CMAKE_TARGET
 
-if [ -n "$VALGRIND" ]; then
+if [ "$VALGRIND" = "yes" ]; then
   # the `--error-exitcode` is required to make the test fail if valgrind found
   # errors, otherwise it'll return 0
   # (http://valgrind.org/docs/manual/manual-core.html)