filter: Fix incorrect filtering with UseFilter context flag and wrapping backends

In this commit

8d1fff9a8237c64f local: obey file filters in listing to fix errors on excluded files

We started using filters in the local backend so the user could short
circuit troublesome files/directories at a low level.

However this caused a number of integration tests to fail. This turned
out to be in backends wrapping the local backend. For example the
combine backend test failed because it changes the paths passed to the
local backend so they no longer match the paths in the current filter.

To fix this, a new feature flag `FilterAware` was added and the
UseFilter context flag is only passed to backends which support it. As
the wrapping backends don't support the flag, this fixes the problems
in the integration tests.

In future the wrapping backends could modify the active filters to
match the path modifications and then they could set the FilterAware
flag.

See #6376

backend/local/local_internal_test.go

@@ -378,6 +378,9 @@ func TestFilter(t *testing.T) {
 	r.WriteFile("excluded", "excluded file", when)
 	f := r.Flocal.(*Fs)
 
+	// Check set up for filtering
+	assert.True(t, f.Features().FilterAware)
+
 	// Add a filter
 	ctx, fi := filter.AddConfig(ctx)
 	require.NoError(t, fi.AddRule("+ included"))