tests: add scrypt benchmark

Times the impact of the parameter "N" to scrypt.

Results on a 2.7GHz Pentium G630:

gocryptfs/cryptfs$ go test -bench=.
PASS
BenchmarkScrypt10-2	     300	   6021435 ns/op ... 6ms
BenchmarkScrypt11-2	     100	  11861460 ns/op
BenchmarkScrypt12-2	     100	  23420822 ns/op
BenchmarkScrypt13-2	      30	  47666518 ns/op
BenchmarkScrypt14-2	      20	  92561590 ns/op ... 92ms
BenchmarkScrypt15-2	      10	 183971593 ns/op
BenchmarkScrypt16-2	       3	 368506365 ns/op
BenchmarkScrypt17-2	       2	 755502608 ns/op ... 755ms
ok  	github.com/rfjakob/gocryptfs/cryptfs	18.772s

cryptfs/kdf_test.go

@@ -0,0 +1,60 @@
+package cryptfs
+
+import (
+	"testing"
+)
+
+/*
+Results on a 2.7GHz Pentium G630:
+
+gocryptfs/cryptfs$ go test -bench=.
+PASS
+BenchmarkScrypt10-2	     300	   6021435 ns/op ... 6ms
+BenchmarkScrypt11-2	     100	  11861460 ns/op
+BenchmarkScrypt12-2	     100	  23420822 ns/op
+BenchmarkScrypt13-2	      30	  47666518 ns/op
+BenchmarkScrypt14-2	      20	  92561590 ns/op ... 92ms
+BenchmarkScrypt15-2	      10	 183971593 ns/op
+BenchmarkScrypt16-2	       3	 368506365 ns/op
+BenchmarkScrypt17-2	       2	 755502608 ns/op ... 755ms
+ok  	github.com/rfjakob/gocryptfs/cryptfs	18.772s
+*/
+
+func benchmarkScryptN(n int, b *testing.B) {
+	kdf := NewScryptKdf(n)
+	for i := 0; i < b.N; i++ {
+		kdf.DeriveKey("test")
+	}
+}
+
+func BenchmarkScrypt10(b *testing.B) {
+	benchmarkScryptN(10, b)
+}
+
+func BenchmarkScrypt11(b *testing.B) {
+	benchmarkScryptN(11, b)
+}
+
+func BenchmarkScrypt12(b *testing.B) {
+	benchmarkScryptN(12, b)
+}
+
+func BenchmarkScrypt13(b *testing.B) {
+	benchmarkScryptN(13, b)
+}
+
+func BenchmarkScrypt14(b *testing.B) {
+	benchmarkScryptN(14, b)
+}
+
+func BenchmarkScrypt15(b *testing.B) {
+	benchmarkScryptN(15, b)
+}
+
+func BenchmarkScrypt16(b *testing.B) {
+	benchmarkScryptN(16, b)
+}
+
+func BenchmarkScrypt17(b *testing.B) {
+	benchmarkScryptN(17, b)
+}