Convert tests/README to markdown and add more information

tests/README

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-Running tests
-=============
-
-You can run all tests using "make check".
-
-Individual tests can be run using ctest.
-For example if the output of "make check" ends like this:
-
-   The following tests FAILED:
-         77 - PyQgsLocalServer (Failed)
-
-You could re-run the failing test with:
-
-   ctest -V -R PyQgsLocalServer
-

tests/README.md

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+QGIS unit tests
+===============
+
+Build tests
+-----------
+
+Make sure that you have enabled building of tests in CMake.
+`cmake -DENABLE_TESTS=ON ..`
+
+Run tests
+---------
+
+You can run all tests using `make check`.
+
+Individual tests can be run using `ctest`.
+
+For example if the output of `make check` ends like this:
+
+
+```
+   The following tests FAILED:
+         77 - PyQgsLocalServer (Failed)
+```
+
+You could re-run the failing test with:
+
+```
+   ctest -V -R PyQgsLocalServer
+```
+
+The parameter `-V` enables verbose mode and `-R` takes a regular expression as
+parameter and will only run matching tests.
+
+Advanced configuration
+----------------------
+
+### Postgres
+
+To test the postgres provider you will need to have a database available to
+which the postgres provider can connect. This will need to have postgis support
+enabled and be available as a service called `qgis_test` on the machine you run
+the tests on.