Introduce the lkt_build_and_run test driver

Transition all relevant testcases to it. In the process, port
docstrings/comments from test.py files to expected_concrete_syntax.lkt
(now test.lkt) sources and fix stylechecks issues there.

(cherry picked from commit e1c4fe5804)

testsuite/tests/python_api/array-struct-array/test.lkt

@@ -1,3 +1,6 @@
+# Check that the Python bindings to wrap/unwrap arrays of structs of arrays work
+# as expected.
+
 import lexer_example
 
 @with_lexer(foo_lexer)

testsuite/tests/python_api/array-struct-array/test.py

@@ -1,35 +0,0 @@
-"""
-Check that the Python bindings to wrap/unwrap arrays of structs of arrays work
-as expected.
-"""
-
-from langkit.dsl import ASTNode, Struct, T, UserField
-from langkit.expressions import Entity, langkit_property
-
-from utils import build_and_run
-
-
-class FooNode(ASTNode):
-    pass
-
-
-class ExampleHolder(Struct):
-    examples = UserField(type=T.Example.entity.array)
-
-
-class Example(FooNode):
-    @langkit_property(return_type=ExampleHolder.array, public=True)
-    def example_holders():
-        return ExampleHolder.new(examples=Entity.singleton).singleton
-
-    @langkit_property(return_type=ExampleHolder.array, public=True)
-    def identity(a=ExampleHolder.array):
-        return a
-
-
-build_and_run(
-    lkt_file='expected_concrete_syntax.lkt',
-    py_script='main.py',
-    types_from_lkt=True,
-)
-print('Done')

testsuite/tests/python_api/array-struct-array/test.yaml

@@ -1 +1,2 @@
-driver: python
+driver: lkt_build_and_run
+py_script: main.py