[Documentation] Define test levels

docs/src/process/testing/plan.md

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+# Test Plan
+
+## Test Levels
+
+Testing occurs regularly during development, with varying levels of
+completeness.
+
+In order of decreasing frequency and increasing completeness, these
+are:
+
+1. __Pre-merge testing__: Performed before changes are integrated
+   into the software.
+2. __Partial acceptance testing__: A subset of acceptance testing
+   performed at regular intervals.
+3. __Acceptance testing__: Performed before a new release is considered
+   stable.
+
+Each level of testing is inclusive of the levels which proceed it.
+