blobs,changefeedccl: append ".tmp" to temp files + deflake TestChange…

…feedNemeses

Prior to this test the blob storage would merely append random numbers
at the end of the file name to generate temp file names.  This was
confusing TestChangefeedNemeses which checks the validity of the
various files in the target directory.

This patch fixes it by using a `.tmp` suffix for temp files. These are
already properly ignored by the walk performed by
TestChangefeedNemeses.

Release note: None

pkg/blobs/local_storage.go

@@ -73,7 +73,9 @@ func (l *LocalStorage) WriteFile(filename string, content io.Reader) (err error)
 	// - it avoids a cross-filesystem rename in the common case.
 	//   (There can still be cross-filesystem renames in very
 	//   exotic edge cases, hence the use fileutil.Move below.)
-	tmpFile, err := ioutil.TempFile(targetDir, filepath.Base(fullPath))
+	// See the explanatory comment for ioutil.TempFile to understand
+	// what the "*" in the suffix means.
+	tmpFile, err := ioutil.TempFile(targetDir, filepath.Base(fullPath)+"*.tmp")
 	if err != nil {
 		return errors.Wrap(err, "creating temporary file")
 	}