Merge #253: cmake: Do not disable libsecp's constant time tests

d2ab006 cmake: Do not disable libsecp's constant time tests (Hennadii Stepanov)

Pull request description:

  This PR addresses #192 (comment):
  > Ok, but the ctime tests are currently compiled, and runnable there.

  and mirrors the behavior of the master branch.

  Assuming the Valgrind is installed, after building the user can tun:
  ```
  $ valgrind ./build/src/secp256k1/src/ctime_tests
  ```

ACKs for top commit:
  TheCharlatan:
    ACK d2ab006

Tree-SHA512: c4dd4967473fecb7297e1b60b02001a08bc4f302266512fcb76e8742e12305b2f3a4aa29a7fb72489fee68548199c9c87e5750ee4184b5eb64f8788083efeef3

src/CMakeLists.txt

@@ -50,7 +50,6 @@ set(SECP256K1_ECMULT_GEN_KB 86 CACHE STRING "" FORCE)
 set(SECP256K1_BUILD_BENCHMARK OFF CACHE BOOL "" FORCE)
 set(SECP256K1_BUILD_TESTS ${BUILD_TESTS} CACHE BOOL "" FORCE)
 set(SECP256K1_BUILD_EXHAUSTIVE_TESTS ${BUILD_TESTS} CACHE BOOL "" FORCE)
-set(SECP256K1_BUILD_CTIME_TESTS OFF CACHE BOOL "" FORCE)
 set(SECP256K1_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)
 include(GetTargetInterface)
 # -fsanitize and related flags apply to both C++ and C,