test: Correct the random seed log on a prevector test failure

rand_cache is unused since commit
16329224e70d0525208f6b0ba00c5e1531a4f5ea, so it can be removed

rand_seed is wrong since commit
022cf47dd7ef8f46e32a184e84f94d1e9f3a495c, because it is no longer
printing the seed that was used to seed the global random context in
tests. Instead, it prints a (random-ish) value derived from the global
random context via InsecureRand256().

Finally, the for loop creating new prevector_tester objects will always
use the same seed since commit fae43a97ca947cd0802392e9bb86d9d0572c0fba,
because repeated calls to SeedInsecureRand/SeedRandomForTest will always
reseed the global with the same "static const" seed.

Fix all issues by
* removing the unused rand_cache,
* removing the call to SeedRandomForTest which restored the same seed on
  every call in the process, and
* Reseeding the global random context with the (random-ish) rand_seed.

src/test/prevector_tests.cpp

@@ -27,7 +27,6 @@ class prevector_tester {
 
     typedef typename pretype::size_type Size;
     bool passed = true;
-    FastRandomContext rand_cache;
     uint256 rand_seed;
 
 
@@ -209,9 +208,8 @@ public:
     }
 
     prevector_tester() {
-        SeedRandomForTest();
         rand_seed = InsecureRand256();
-        rand_cache.Reseed(rand_seed);
+        g_insecure_rand_ctx.Reseed(rand_seed);
     }
 };