Relocate the clock time getter to tests_common.h to
make it easily reusable across test programs. This
will be useful for the upcoming unit test framework.
Context - why not placing it inside testutil.h?:
The bench program links against the production-compiled library,
not its own compiled version. Therefore, `gettime_i64()` cannot
be moved to testutil.h, because testutil.h calls
`secp256k1_pubkey_save()`, which exists only in the internal
secp256k1.c and not in the public API.
