Enable support for C++ modules tests

Modules-specific tests can be written under respective module folders.
Each module should have "tests" folder created with the tests implemented
as `doctest` headers, so they can be collected by the buildsystem and
included directly in `tests/test_main.cpp` to be compiled.

modules/SCsub

@@ -1,10 +1,10 @@
 #!/usr/bin/env python
 
-Import("env")
-
 import modules_builders
 import os
 
+Import("env")
+
 env_modules = env.Clone()
 
 Export("env_modules")
@@ -12,6 +12,15 @@ Export("env_modules")
 # Header with MODULE_*_ENABLED defines.
 env.CommandNoCache("modules_enabled.gen.h", Value(env.module_list), modules_builders.generate_modules_enabled)
 
+# Header to be included in `tests/test_main.cpp` to run module-specific tests.
+if env["tests"]:
+    env.CommandNoCache(
+        "modules_tests.gen.h",
+        Value(env.module_list),
+        Action(modules_builders.generate_modules_tests, "Generating modules tests header."),
+    )
+    env.AlwaysBuild("modules_tests.gen.h")
+
 vs_sources = []
 # libmodule_<name>.a for each active module.
 for name, path in env.module_list.items():