MDL-67673 phpunit: Document all the changes in upgrade.txt

This includes:

- Breaking changes (return void + PHP 7.1 min requirement).
- Deprecations and removals.
- Specifically DbUnit and its replacement.
- Printer showing how to rerun tests removed.

lib/upgrade.txt

@@ -3,6 +3,27 @@ information provided here is intended especially for developers.
 
 === 3.10 ===
 * PHPUnit has been upgraded to 8.5. That comes with a few changes:
+  - Breaking change: All the "template methods" (setUp(), tearDown()...) now require to return void. This implies
+    that the minimum version of PHP able to run tests will be PHP 7.1
+  - A good number of assertions have been deprecated with this version
+    and will be removed in a future one. In core all cases have been removed
+    (so it's deprecation-warnings free). It's recommended to perform the
+    switch to their new counterparts ASAP:
+      - assertInternalType() has been deprecated. Use the assertIsXXX() methods instead.
+      - assertArraySubset() has been deprecated. Use looping + assertArrayHasKey() or similar.
+      - @expectedExceptionXXX annotations have been deprecated. Use the expectExceptionXXX()
+        methods instead (and put them exactly before the line that is expected to throw the exception).
+      - assertAttributeXXX() have been deprecated. If testing public attributes use normal assertions. If
+        testing non-public attributes... you're doing something wrong :-)
+      - assertContains() to find substrings on strings has been deprecated. Use assertStringContainsString() instead.
+        (note that there are "IgnoringCase()" variants to perform case-insensitive matching.
+      - assertEquals() extra params have been deprecated and new assertions for them created:
+        - delta => use assertEqualsWithDelta()
+        - canonicalize => use assertEqualsCanonicalizing()
+        - ignoreCase => use assertEqualsIgnoringCase()
+        - maxDepth => removed without replacement.
+  - The custom printer that was used to show how to rerun a failure has been removed, it was old and "hacky"
+    solution, for more information about how to run tests, see the docs, there are plenty of options.
   - phpunit/dbunit is not available any more and it has been replaced by a lightweight phpunit_dataset class, able to
     load XML/CSV and PHP arrays, send the to database and return rows to calling code (in tests). That implies the
     follwoing changes in the advanced_testcase class: