Update cheatsheets

tldr/pg_test_timing

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pg_test_timing
+
+> Measure timing overhead on your system (ensure clock monotonicity).
+> More information: <https://www.postgresql.org/docs/current/pgtesttiming.html>.
+
+- Run the default timing test (3 seconds):
+
+`pg_test_timing`
+
+- Run for a custom duration:
+
+`pg_test_timing {{[-d|--duration]}} {{seconds}}`