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.

types/genesis_test.go

@@ -126,7 +126,7 @@ func TestBasicGenesisDoc(t *testing.T) {
 }
 
 func TestGenesisSaveAs(t *testing.T) {
-	tmpfile, err := os.CreateTemp("", "genesis")
+	tmpfile, err := os.CreateTemp(t.TempDir(), "genesis")
 	require.NoError(t, err)
 	defer os.Remove(tmpfile.Name())