docs(tests): mention performance tests in TESTPLAN.md #5793

Author: Hweinstock

docs/TESTPLAN.md

@@ -42,6 +42,11 @@ The test suite has the following categories of tests:
     -   Live in `src/testE2E`
     -   These tests are heavier than Integration tests.
     -   Some E2E tests have a more complicated architecture, described in [TEST_E2E](./TEST_E2E.md)
+-   Performance Tests: **slow** tests
+    -   Live in `src/testInteg/perf`.
+    -   A subset of integration tests focused on catching performance regressions.
+    -   Use a combination of operation counting and system usage statistics such as cpu usage, memory usage, and duration.
+    -   Each test is often repeated 10 or more times for less variant system usage statistics, then median of runs is used.
 
 ## Test files