Clean up temp files more thoroughly after testing. (#7815)

Our test cases spew a lot of files and directories around $TMPDIR.  Make more
thorough use of the testing package's TempDir methods to ensure these are
cleaned up.

In a few cases, this required plumbing test contexts through existing helper
code. In a couple places an explicit path was required, to work around cases
where we do global setup during a TestMain function. Those cases probably
deserve more thorough cleansing (preferably with fire), but for now I have just
worked around it to keep focused on the cleanup.

light/provider/http/http_test.go

@@ -35,7 +35,7 @@ func TestNewProvider(t *testing.T) {
 func TestProvider(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	cfg, err := rpctest.CreateConfig(t.Name())
+	cfg, err := rpctest.CreateConfig(t, t.Name())
 	require.NoError(t, err)
 
 	// start a tendermint node in the background to test against