Add a README.txt to explain the testing organization.

test/README.txt

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+Overview of CUDA Quantum Testing
+--------------------------------
+
+There are several directories in the repo related to testing. Please make sure
+that tests conform to the following general hierarchy when writing tests and
+reviewing PRs.
+
+test
+
+  The tests in this directory tree are small regression tests that can be run
+  very quickly and test small combinations of functionality to prevent
+  regressions. These tests use shell script command lines, various cudaq tools,
+  FileCheck, etc.
+
+unittests
+
+  As the name implies, these are unit tests. Like the test directory, these
+  tests are expected to run quickly and test small scopes of functionality.
+  Unit tests may test boundary conditions, out-of-range values, etc. to
+  verify that the software unit is behaving according to its specification.
+  These tests make use of the gtest framework.
+
+targettests
+
+  The tests in this directory are end-to-end tests to test the complete path of
+  functionality from source to a QPU target, which may be a specific simulator
+  or hardware. These tests may take a much longer time to execute and will
+  often test multiple tools in the ecosystem. They may even depend on network
+  interfaces being operational, etc.
+
+  Note: At some future point in time, the tests in targettests will be used to
+  test out-of-tree on CUDA Quantum installations. Such installations will not
+  have the source code around. Be mindful of writing tests that use the content
+  or paths of other files in other areas of the repo.
+
+python/tests
+
+  The python tests, for historical reasons, are broken out into their own
+  directory at present. CUDA Quantum support for the python language continues
+  to develop and improve. At some point, CUDA Quantum will integrate both C++ and
+  python and these tests will be merged with and into the 3 directories explained
+  above. In the meantime, python specific tests will continue to be placed in
+  the python/tests directory.