runtimetest: Use ModeType as the mask in the symlink check

We've been using ModeSymlink as the mask since the check landed in
da25004 (runtimetest: add linux default symbolic link validation,
2016-11-30, #284).  POSIX provides S_IS*(m) macros to portably
interpret the mode type, but does not define values for each type [2].
Alban pointed out that st_mode is not a bitfield on Linux [1].  For
example, Linux defines [3]:

  S_IFBLK 060000
  S_IFDIR 040000
  S_IFCHR 020000

So 'm&S_IFCHR == S_IFCHR', for example, would succeed for both
character and block devices.  Go translates the system values to a
platform-agnostic bitfield [4], so the previous approach works on Go.
But it may be confusing for people used to the native non-bitfield
mode, so this commit moves us to an approach that does not rely on
Go's using a bitfield.

[1]: #308 (comment)
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_stat.h.html
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/stat.h?h=v4.16#n9
[4]: https://github.com/golang/go/blob/b0d437f866eb8987cde7e6550cacd77876f36d4b/src/os/types.go#L45

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

cmd/runtimetest/main.go

@@ -782,7 +782,7 @@ func (c *complianceTester) validateDefaultSymlinks(spec *rspec.Spec) error {
 			continue
 		}
 
-		isSymlink := fi.Mode()&os.ModeSymlink == os.ModeSymlink
+		isSymlink := fi.Mode()&os.ModeType == os.ModeSymlink
 		rfcError, err = c.Ok(
 			isSymlink,
 			specerror.DefaultRuntimeLinuxSymlinks,