tests: don't read /proc, the number of entries changes too quickly
tests: don't read /proc, the number of entries changes too quickly

This could lead to test failures like this:

  --- FAIL: TestGetdents (0.02s)
  	getdents_test.go:57: len(getdentsEntries)=362, len(readdirEntries)=360
  FAIL

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs