fix(epictest) generate data before starting benchmark

[btc]

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