Add note on ASAN/UBSAN affecting test performance

This came up while submitting rpm-software-management#3540.

(backported from commit ad33670)

tests/README.md

@@ -156,3 +156,14 @@ as well as the existing tests.  Below are the specifics of RPM's test-suite:
       `runroot_user -n <name>` to run a binary as a specific user
 * If no snapshot was used, just call the RPM binaries normally
 * Store any working files in the current directory (it's always writable)
+
+### Tips & Tricks
+
+* If you've configured your CMake build with the `ENABLE_ASAN` and/or
+  `ENABLE_UBSAN` options enabled (OFF by default), the test-suite will run
+  significantly slower, increasing the total execution time by as much as 2
+  minutes on modern hardware.  If this is an issue, consider setting up a
+  separate CMake build on the side without these options enabled and use that
+  for running the test-suite iteratively.  Before submitting a PR, though,
+  please verify that the test-suite still passes with those options enabled
+  (the easiest way is to run `make ci`).