Re-revert and edit the info about test suffixes

Author: Kordyjan

docs/_docs/contributing/testing.md

@@ -127,6 +127,14 @@ $ sbt
 > testCompilation --help
 ```
 
+### Joint and separate sources compilation
+
+When the sources of a test consist of multiple source files places in a single directory they are passed to the compiler in a single run and the compiler decides in which order to compile them. In some cases, however, to reproduce a specific test scenario it might be necessary to compile the source files in several steps in a specified order. To achieve that one can add a `_${step_index}` suffix to a file name (before the `.scala` or `.java` extension) indicating the order of compilation. E.g. if the test directory contains files named `Foo_1.scala`, `Bar_2.scala` and `Baz_2.scala` then `Foo_1.scala` will be compiled first and after that `Bar_2.scala` together with `Baz_2.scala`.
+
+The other kind of suffix that can modify how particular files are compiled is `_c${compilerVersion}`. When specified, the file will be compiled with a specific version of the compiler instead of the one developed on the current branch.
+
+Different suffixes can be mixed together (their order is not important although consistency is advised), e.g. `Foo_1_c3.0.2`, `Bar_2_c3.1.0`.
+
 ### Bootstrapped-only tests
 
 To run `testCompilation` on a bootstrapped Dotty compiler, use