[tx_test.py] change skip regex for bug1218 to skip date & time metrics

[cjchapman]

This change is intended to fix this issue:

--- /home/runner/work/afdko/afdko/tests/tx_data/expected_output/bug1218.pdf
+++ /tmp/tmppqy3jwgv/bug1218.pdf
@@ -36,5 +36,5 @@
 (Em:  4000 units)'
-498.50 30.00 Td
+500.30 30.00 Td
 
-16.00 0.00 Td
+14.10 0.00 Td

Here's the relevant section of the PDF, for context:

(Filename:  bug1218.otf)'
(FontName:  UPM-Regular)'
(Em:  4000 units)'
500.30 30.00 Td
(Date:  02 Oct 20)'
14.10 0.00 Td
(Time:  13:49)'

[punchcutter]

lgtm