sysfs: adds FS.Stat and companions in platform (#1140)

This centralizes filestat logic by making our own `Stat_t` similar to
`syscall.Stat_t`. This exposes utilities in the platform package and
adds a new function `FS.Stat` which avoids having to use `fs.File` to
get the same info. Doing so at the FS abstraction allows us to optimize
how it is implemented internally using portable means (e.g.
`os.StatFile`) or OS-specific means where necessary, e.g. in windows.

This also ensures `platform.OpenFile` returns syscall.Errno and
centralizes error checking with a new `require.EqualErrno` test.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/testing/require/require.go

@@ -13,6 +13,7 @@ import (
 	"reflect"
 	"runtime"
 	"strings"
+	"syscall"
 	"unicode"
 	"unicode/utf8"
 )
@@ -126,6 +127,19 @@ func Error(t TestingT, err error, formatWithArgs ...interface{}) {
 	}
 }
 
+// EqualErrno should be used for functions that return syscall.Errno or nil.
+func EqualErrno(t TestingT, expected syscall.Errno, err error, formatWithArgs ...interface{}) {
+	if err == nil {
+		fail(t, "expected a syscall.Errno, but was nil", "", formatWithArgs...)
+		return
+	}
+	if se, ok := err.(syscall.Errno); !ok {
+		fail(t, fmt.Sprintf("expected %v to be a syscall.Errno", err), "", formatWithArgs...)
+	} else if se != expected {
+		fail(t, fmt.Sprintf("expected Errno %#[1]v(%[1]s), but was %#[2]v(%[2]s)", expected, err), "", formatWithArgs...)
+	}
+}
+
 // ErrorIs fails if the err is nil or errors.Is fails against the expected.
 //
 //   - formatWithArgs are optional. When the first is a string that contains '%', it is treated like fmt.Sprintf.