Replace uses of ClangEndian with a NativeFile test utility.

Signed-off-by: Lorenz Bauer <[email protected]>

We have a lot of tests which first glob a list of files containing BPF for
little and big endian, then parse those files and finally skip a test if the
files endianness doesn't match the native endianness.

This is pretty wasteful, since we parse BPF for no good reason and makes the
tests more verbose than they have to be. Use NativeFile instead and remove
almost all checks for IsNativeEndian from tests.

The two remaining cases are in the ELF loader tests, where they do have
value: we want to make sure that we can parse things from any endianness.

Signed-off-by: Lorenz Bauer <[email protected]>