build: add pytest targets

It enables running pytests in cmake jobs, regardless of underlying build
tool choice (= makes it work with ninja.)

Also:

- drop pytest logic launching `make` and exiting in case of failure.
  Maybe there is a better way and keep this functionality somehow, bind
  it to a command-line option? make it fail softly?

- GHA/linux: invoke pytest via the build, not directly.

- autotools: add missing dummy runtests targets when cross-compiling.

Closes #15034

Makefile.am

@@ -118,7 +118,13 @@ check: test examples check-docs
 
 if CROSSCOMPILING
 test-full: test
+test-nonflaky: test
 test-torture: test
+test-event: test
+test-am: test
+test-ci: test
+pytest: test
+pytest-ci: test
 
 test:
 	@echo "NOTICE: we can't run the tests when cross-compiling!"
@@ -146,6 +152,12 @@ test-am:
 test-ci:
 	@(cd tests; $(MAKE) all ci-test)
 
+pytest:
+	@(cd tests; $(MAKE) all default-pytest)
+
+pytest-ci:
+	@(cd tests; $(MAKE) all ci-pytest)
+
 endif
 
 examples: