Add a basic testing README including normalization explanation

src/test/regress/README.md

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+# How our testing works
+
+We use the test tooling of postgres to run our tests. This tooling is very
+simple but effective. The basics it runs a series of `.sql` scripts, gets
+their output and stores that in `results/$sqlfilename.out`. It then compares the
+actual output to the expected output with a simple `diff` command:
+
+```bash
+diff results/$sqlfilename.out expected/$sqlfilename.out
+```
+
+## Schedules
+
+Which sql scripts to run is defined in a schedule file, e.g. `multi_schedule`,
+`multi_mx_schedule`.
+
+## Makefile
+
+In our `Makefile` we have rules to run the different types of test schedules.
+You can run them from the root of the repository like so:
+```bash
+# e.g. the multi_schedule
+make install -j9 && make -C src/test/regress/ check-multi
+```
+
+Take a look at the makefile for a list of all the testing targets.
+
+### Running a specific test
+
+Often you want to run a specific test and don't want to run everything. You can
+use one of the following commands to do so:
+```bash
+# If your tests needs almost no setup you can use check-minimal
+make install -j9 && make -C src/test/regress/ check-minimal EXTRA_TESTS='multi_utility_warnings'
+# Often tests need some testing data, if you get missing table errors using
+# check-minimal you should try check-base
+make install -j9 && make -C src/test/regress/ check-base EXTRA_TESTS='with_prepare'
+# Sometimes this is still not enough and some other test needs to be run before
+# the test you want to run. You can do so by adding it to EXTRA_TESTS too.
+make install -j9 && make -C src/test/regress/ check-base EXTRA_TESTS='add_coordinator coordinator_shouldhaveshards'
+```
+
+## Normalization
+
+The output of tests is sadly not completely predictable. Still we want to
+compare the output of different runs and error when the important things are
+different. We do this by not using the regular system `diff` to compare files.
+Instead we use `src/test/regress/bin/diff` which does the following things:
+
+1. Change the `$sqlfilename.out` file by running it through `sed` using the
+   `src/test/regress/bin/normalize.sed` file. This does stuff like replacing
+   numbers that keep changing across runs with an `XXX` string, e.g. portnumbers
+   or transaction numbers.
+2. Backup the original output to `$sqlfilename.out.unmodified` in case it's
+   needed for debugging
+3. Compare the changed `results` and `expected` files with the system `diff`
+   command.
+
+
+## Updating the expected test output
+
+Sometimes you add a test to an existing file, or test output changes in a way
+that's not bad (possibly even good if support for queries is added). In those
+cases you want to update the expected test output.
+The way to do this is very simple, you run the test and copy the new .out file
+in the `results` directory to the `expected` directory, e.g.:
+
+```bash
+make install -j9 && make -C src/test/regress/ check-minimal EXTRA_TESTS='multi_utility_warnings'
+cp src/test/regress/{results,expected}/multi_utility_warnings.out
+```
+
+
+## Adding a new test file
+
+Adding a new test file is quite simple:
+
+1. Write the SQL file in the `sql` directory
+2. Add it to a schedule file, to make sure it's run in CI
+3. Run the test
+4. Check that the output is as expected
+5. Copy the `.out` file from `results` to `expected`
+
+## Isolation testing
+See [`src/test/regress/spec/README.md`](https://github.com/citusdata/citus/blob/master/src/test/regress/spec/README.md)
+
+## Upgrade testing
+See [`src/test/regress/upgrade/README.md`](https://github.com/citusdata/citus/blob/master/src/test/regress/upgrade/README.md)
+
+## Failure testing
+
+See [`src/test/regress/mitmscripts/README.md`](https://github.com/citusdata/citus/blob/master/src/test/regress/mitmscripts/README.md)
+
+## Perl test setup script
+
+To automatically setup a citus cluster in tests we use our
+`src/test/regress/pg_regress_multi.pl` script. This sets up a citus cluster and
+then starts the standard postgres test tooling. You almost never have to change
+this file.