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

cmd/tools/modules/testing/common.v

@@ -358,12 +358,19 @@ pub fn prepare_test_session(zargs string, folder string, oskipped []string, main
 	mut mains := []string{}
 	mut skipped := oskipped.clone()
 	next_file: for f in files {
-		if f.contains('modules') || f.contains('preludes') {
+		fnormalised := f.replace('\\', '/')
+		// NB: a `testdata` folder, is the preferred name of a folder, containing V code,
+		// that you *do not want* the test framework to find incidentally for various reasons,
+		// for example module import tests, or subtests, that are compiled/run by other parent tests
+		// in specific configurations, etc.
+		if fnormalised.contains('testdata/') || fnormalised.contains('modules/')
+			|| f.contains('preludes/') {
 			continue
 		}
 		$if windows {
 			// skip pico and process/command examples on windows
-			if f.ends_with('examples\\pico\\pico.v') || f.ends_with('examples\\process\\command.v') {
+			if fnormalised.ends_with('examples/pico/pico.v')
+				|| fnormalised.ends_with('examples/process/command.v') {
 				continue
 			}
 		}

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,