misc/cgo/testgodefs: move source files into testdata

These source files fail to build with 'go test ./...'.
Move them into testdata so that only test.bash will see them.

Updates #30228

Change-Id: I3673f3cb64b0c128a2bca5fee7679b672fe90770
Reviewed-on: https://go-review.googlesource.com/c/163212
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testgodefs/test.bash

@@ -9,6 +9,8 @@
 # import "C" block.  Add more tests here.
 FILE_PREFIXES="anonunion issue8478 fieldtypedef"
 
+cd testdata
+
 RM=
 for FP in $FILE_PREFIXES
 do
@@ -16,7 +18,7 @@ do
   RM="${RM} ${FP}_defs.go"
 done
 
-go build . && ./testgodefs
+go build -o testgodefs . && ./testgodefs
 EXIT=$?
 rm -rf _obj testgodefs ${RM}
 exit $EXIT