tests: ignore testdata folders while scanning for tests and .v files

doc/docs.md

@@ -3219,6 +3219,25 @@ To test an entire module, use `v test mymodule`. You can also use `v test .` to
 everything inside your current folder (and subfolders). You can pass the `-stats`
 option to see more details about the individual tests run.
 
+You can put additional test data, including .v source files in a folder, named
+`testdata`, right next to your _test.v files. V's test framework will *ignore*
+such folders, while scanning for tests to run. This is usefull, if you want to
+put .v files with invalid V source code, or other tests, including known 
+failing ones, that should be run in a specific way/options by a parent _test.v 
+file.
+
+NB: the path to the V compiler, is available through @VEXE, so a _test.v
+file, can easily run *other* test files like this:
+```v oksyntax
+import os
+
+fn test_subtest() {
+	res := os.execute('${@VEXE} other_test.v')
+	assert res.exit_code == 1
+	assert res.output.contains('other_test.v does not exist')
+}
+```
+
 ## Memory management
 
 V avoids doing unnecessary allocations in the first place by using value types,