tests: Add file create benchmarks for different sizes

Example output on my machine:

~/src/github.com/rfjakob/gocryptfs$ ./benchmark.bash
gocryptfs v0.3.1-25-g2e33888-dirty; on-disk format 2
PASS
BenchmarkStreamWrite-2	     100	  12189867 ns/op	  86.02 MB/s
BenchmarkStreamRead-2 	     200	   9113262 ns/op	 115.06 MB/s
BenchmarkCreate0B-2   	   10000	    100249 ns/op
BenchmarkCreate1B-2   	   10000	    177680 ns/op	   0.01 MB/s
BenchmarkCreate100B-2 	    3000	    431586 ns/op	   0.23 MB/s
BenchmarkCreate4kB-2  	    3000	    455204 ns/op	   9.00 MB/s
BenchmarkCreate10kB-2 	    3000	    594044 ns/op	  17.24 MB/s
ok  	github.com/rfjakob/gocryptfs/integration_tests	15.176s

0 files changed, 0 insertions, 0 deletions