fix(epictest) generate data before starting benchmark

for faster, more accurate benchmark results

```
+ go test ./... -bench=. -v -run=Bench
PASS
BenchmarkInstantaneousAddCat1MB	      50	  20434373 ns/op	  51.31 MB/s
BenchmarkInstantaneousAddCat2MB	     100	  26463847 ns/op	  79.25 MB/s
BenchmarkInstantaneousAddCat4MB	      50	  52444540 ns/op	  79.98 MB/s
BenchmarkInstantaneousAddCat8MB	      20	 125603062 ns/op	  66.79 MB/s
BenchmarkInstantaneousAddCat16MB	       5	 285122576 ns/op	  58.84 MB/s
BenchmarkInstantaneousAddCat32MB	       5	 577644364 ns/op	  58.09 MB/s
BenchmarkInstantaneousAddCat64MB	       1	1469263280 ns/op	  45.68 MB/s
BenchmarkInstantaneousAddCat128MB	       1	2174235476 ns/op	  61.73 MB/s
BenchmarkInstantaneousAddCat256MB	       1	4819592739 ns/op	  55.70 MB/s
BenchmarkRoutingSlowAddCat1MB	       1	2617028905 ns/op	   0.40 MB/s
BenchmarkRoutingSlowAddCat2MB	       1	4652568963 ns/op	   0.45 MB/s
BenchmarkRoutingSlowAddCat4MB	       1	7460396361 ns/op	   0.56 MB/s
BenchmarkRoutingSlowAddCat8MB	       1	12527218436 ns/op	   0.67 MB/s
BenchmarkRoutingSlowAddCat16MB	       1	18445741866 ns/op	   0.91 MB/s
BenchmarkRoutingSlowAddCat32MB	       1	36389865187 ns/op	   0.92 MB/s
BenchmarkNetworkSlowAddCat1MB	      20	 112103824 ns/op	   9.35 MB/s
BenchmarkNetworkSlowAddCat2MB	      10	 131731348 ns/op	  15.92 MB/s
BenchmarkNetworkSlowAddCat4MB	      10	 188635880 ns/op	  22.23 MB/s
BenchmarkNetworkSlowAddCat8MB	       5	 275349889 ns/op	  30.47 MB/s
BenchmarkNetworkSlowAddCat16MB	       5	 328141138 ns/op	  51.13 MB/s
BenchmarkNetworkSlowAddCat32MB	       5	 454094845 ns/op	  73.89 MB/s
BenchmarkNetworkSlowAddCat64MB	       2	 801813609 ns/op	  83.70 MB/s
BenchmarkNetworkSlowAddCat128MB	       1	1793868067 ns/op	  74.82 MB/s
BenchmarkNetworkSlowAddCat256MB	       1	3586404768 ns/op	  74.85 MB/s
BenchmarkBlockstoreSlowAddCat1MB	      20	 123439675 ns/op	   8.49 MB/s
BenchmarkBlockstoreSlowAddCat2MB	      10	 211015893 ns/op	   9.94 MB/s
BenchmarkBlockstoreSlowAddCat4MB	       5	 398302066 ns/op	  10.53 MB/s
BenchmarkBlockstoreSlowAddCat8MB	       2	 845097118 ns/op	   9.93 MB/s
BenchmarkBlockstoreSlowAddCat16MB	       1	1707422430 ns/op	   9.83 MB/s
BenchmarkBlockstoreSlowAddCat32MB	       1	3319987644 ns/op	  10.11 MB/s
BenchmarkBlockstoreSlowAddCat64MB	       1	5960528012 ns/op	  11.26 MB/s
BenchmarkBlockstoreSlowAddCat128MB	       1	11982092666 ns/op	  11.20 MB/s
BenchmarkBlockstoreSlowAddCat256MB	       1	19802131338 ns/op	  13.56 MB/s
ok  	github.com/jbenet/go-ipfs/epictest	215.045s
```