Add forgotten test_decoder.c.

tests/test_decoder.c

@@ -0,0 +1,32 @@
+
+#include "upb_decoder.h"
+#include "upb_textprinter.h"
+#include "upb_stdio.h"
+
+int main() {
+  upb_symtab *symtab = upb_symtab_new();
+  upb_symtab_add_descriptorproto(symtab);
+  upb_def *fds = upb_symtab_lookup(
+      symtab, UPB_STRLIT("google.protobuf.FileDescriptorSet"));
+
+  upb_stdio *in = upb_stdio_new();
+  upb_stdio_reset(in, stdin);
+  upb_stdio *out = upb_stdio_new();
+  upb_stdio_reset(out, stdout);
+  upb_decoder *d = upb_decoder_new(upb_downcast_msgdef(fds));
+  upb_decoder_reset(d, upb_stdio_bytesrc(in));
+  upb_textprinter *p = upb_textprinter_new();
+  upb_textprinter_reset(p, upb_stdio_bytesink(out), false);
+
+  upb_status status = UPB_STATUS_INIT;
+  upb_streamdata(upb_decoder_src(d), upb_textprinter_sink(p), &status);
+
+  assert(upb_ok(&status));
+
+  upb_stdio_free(in);
+  upb_stdio_free(out);
+  upb_decoder_free(d);
+  upb_textprinter_free(p);
+  upb_def_unref(fds);
+  upb_symtab_unref(symtab);
+}