Add instructions on how to run regress tests and how to audit a new regress tests

[safern]

We should add a documentation on how to run and audit regress tests. Also a brief overview of how they work.

Here's a summary of how to audit a new test (this needs to be cleaned up):

This is a regress test for example:
https://github.com/microsoft/documentdb/blob/main/pg_documentdb/src/test/regress/sql/commands_delete.sql

You need to add a new sql file on that directory. Then add it to the schedule here:
https://github.com/microsoft/documentdb/blob/main/pg_documentdb/src/test/regress/basic_schedule

You can run the individual test with:

make check-minimal -C pg_documentdb EXTRA_TESTS=<test-file-without-extension

You will see a test failure, because there is no expected .out file, so an empty one will be created in this directory:
https://github.com/microsoft/documentdb/tree/main/pg_documentdb/src/test/regress/expected

If the output of your test looks as expected, copy the contents from:
pg_documentdb/src/test/regress/results/<your_test.out> into the expected .out file.

To run all tests just do:

make check

To get the next collection id to use at the top of your test file after running make check you can run this script:
https://github.com/microsoft/documentdb/blob/main/pg_documentdb/src/test/regress/validate_test_output.sh

[lichoil]

@SelinaTang21 will be working on this documentation.

[SelinaTang21]

I’ve added a preliminary guide based on the procedure @safern provided! https://github.com/microsoft/documentdb/wiki/Regression-Tests-Guide Waiting for PM's review.