Remove timing-based signature cache unit test

Two changes:

First removes a unit test that fails in my development environment
(OSX, compiled -g3 with clang).

sipa says that's not terribly surprising; the CMutableTransaction change
makes signing a little more expensive but verification quicker. The unit
test timed sign+verify-uncached versus verify-cached-five-times.

He also says the test will be invalid when libsec256kp1 is integrated
(because validation is super-optimized over signing).

core.h change fixes a compiler warning (clang -Wall : CMutableTransaction defined
as struct, declared as class in script.h).

src/script.h

@@ -20,7 +20,7 @@
 class CCoins;
 class CKeyStore;
 class CTransaction;
-class CMutableTransaction;
+struct CMutableTransaction;
 
 static const unsigned int MAX_SCRIPT_ELEMENT_SIZE = 520; // bytes
 static const unsigned int MAX_OP_RETURN_RELAY = 40;      // bytes