Tests: Add testing overview documentation

Change-Id: I37ca6b6207b9da8653331b367fb66ba147490101
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

TESTING.md

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+# Testing Qt Creator
+
+There are four kinds of tests in Qt Creator, you can find some more documentation
+[online here](https://doc-snapshots.qt.io/qtcreator-extending/plugin-tests.html).
+
+## Manual Tests
+
+Located in `tests/manual/...`, these need to be run manually as separate executables and require
+user interaction for "playing around" with some piece of code.
+
+## Squish-Based Tests
+
+`tests/system/...` are run with [Squish](https://www.qt.io/product/quality-assurance/squish)
+at irregular intervals and specifically checked for releases (RTA).
+
+## Auto/Unit Tests
+
+`tests/auto/...` are separate QTest-based executables (tst_...) that are run with tools like
+cmake/ctest. They run automatically with the PRECHECK in Gerrit, etc.
+
+## Plugin/Integration Tests
+
+These tests are integrated into the plugins themselves and are executed within
+the context of a fully running Qt Creator. They are run with the `-test <plugin>`
+command line parameter of the qtcreator executable.
+This starts Qt Creator with only that plugin and its dependencies loaded, executes any test
+functions that the plugin registers, prints the QTest output, and then closes.
+
+For plugin tests, plugins register test functions if Qt Creator was compiled with WITH_TESTS,
+like this:
+
+```c++
+void TextEditorPlugin::initialize()
+{
+#ifdef WITH_TESTS
+    addTestCreator(createFormatTextTest);
+    addTestCreator(createTextDocumentTest);
+    addTestCreator(createTextEditorTest);
+    addTestCreator(createSnippetParserTest);
+#endif
+    ...
+}
+```
+
+The "test creator" is a factory function that returns a QObject, which functions the same as the
+tstSomething QObjects used in standalone unit tests.
+
+```c++
+QObject *createFormatTextTest()
+{
+    return new FormatTextTest;
+}
+```
+
+Slots are executed as QTest tests, and there can be init and cleanup functions, etc.
+
+```c++
+class FormatTextTest final : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void testFormatting_data();
+    void testFormatting();
+};
+```
+
+These tests are executed after Qt Creator plugins have fully initialized and can access the full
+Qt Creator state, open files, load projects, etc. (depending on the declared dependencies of
+the tested plugin).