Manual makefile does not clean fypp generated test files

[awvwgk]

The test_median is currently using fypp to generate the actual test file:

stdlib/src/tests/stats/Makefile.manual

Lines 1 to 7 in 8bfcdf9

	SRCFYPP =\
	test_median.fypp
	SRCGEN = $(SRCFYPP:.fypp=.f90)
	$(SRCGEN): %.f90: %.fypp common.fypp
	fypp $(FYPPFLAGS) $< $@

Since the include makefile for generating the tests is not aware of the fypp usage the clean command is not working correctly. Running the manual makefile will leave a stale build artifact in the source tree which can be picked when doing a local fpm export using the ci/fpm-deployment.sh script or by the CMake build files.

[ivan-pi]

Closing due to #657 (Remove support for manual make builds).