internal/encoding/yaml/goccy: copy decoder tests from internal/encoding/yaml

Copy decode_test.go and its testdata verbatim into the new goccy
package as the baseline for the test suite of the goccy based decoder
that the next change introduces, so that the adjustments the new
decoder needs are visible as ordinary diffs rather than buried in a
new file. The copy carries a build constraint that excludes it from
builds until that change adapts it; the original tests keep running
in internal/encoding/yaml.

Signed-off-by: Matthew Sackman <matthew@cue.works>
Change-Id: Ia98158323320f7655765e19d1559b084a4c81017
Assisted-by: Claude Code (claude-fable-5)
Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1243969
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

Author: cuematthew