Fix hashtree:sha_test_ from timing out

EQC takes a long time, and uses gigabytes of memory to generate 1MB
random binaries, as evidenced by:

    eqc_gen:sample(eqc_gen:binary(1024 * 1024)).

We're still able to write a solid test, by ensuring that we always chunk
the binary into between 1 and 16 chunks. So instead of generating the
chunk size, we now generate the number of chunks, and derive chunk size
from that. This change allowed us to remove one level of ?FORALL
nesting, as well.

src/hashtree.erl

@@ -1129,10 +1129,18 @@ sha_test_() ->
     }.
 
 prop_sha() ->
-    ?FORALL(Size, choose(256, 1024*1024),
-            ?FORALL(Chunk, choose(1, Size),
+    %% NOTE: Generating 1MB (1024 * 1024) size binaries is incredibly slow
+    %% with EQC and was using over 2GB of memory
+    ?FORALL({Size, NumChunks}, {choose(1, 1024), choose(1, 16)},
                     ?FORALL(Bin, binary(Size),
-                            sha(Chunk, Bin) =:= esha(Bin)))).
+                            begin
+                                %% we need at least one chunk,
+                                %% and then we divide the binary size
+                                %% into the number of chunks (as a natural
+                                %% number)
+                                ChunkSize = max(1, (Size div NumChunks)),
+                                sha(ChunkSize, Bin) =:= esha(Bin)
+                            end)).
 
 eqc_test_() ->
     {timeout, 5,