Makefile.include: add a 'test/available' target

This allows querying the build system if there are test available.

Before, one should rely on 'info-debug-variable-TESTS' to print the list
of test files. But was not reliable as sometime the build system printed
messages anyway.

    BOARD=esp32-wroom-32 make --silent --no-print-directory \
        -C examples/hello-world/ info-debug-variable-TESTS
    ESP32_SDK_DIR should be defined as /path/to/esp-idf directory
    ESP32_SDK_DIR is set by default to /opt/esp/esp-idf
    # empty line here

Now the return code can be trusted.

Makefile.include

@@ -569,7 +569,7 @@ reset:
 	$(call check_cmd,$(RESET),Reset program)
 	$(RESET) $(RESET_FLAGS)
 
-.PHONY: test
+.PHONY: test test/available
 TESTS ?= $(foreach file,$(wildcard $(APPDIR)/tests/*),\
                         $(shell test -f $(file) -a -x $(file) && echo $(file)))
 test: $(TEST_DEPS)
@@ -577,6 +577,10 @@ test: $(TEST_DEPS)
 		$$t || exit 1; \
 	done
 
+test/available:
+	$(Q)test -n "$(strip $(TESTS))"
+
+
 # Default OBJDUMPFLAGS for platforms which do not specify it:
 OBJDUMPFLAGS ?= -S -D -h
 

tests/README.md

@@ -20,3 +20,10 @@ testing.
 From the test application directory run:
 
     BOARD=<board_of_your_choice> make flash test
+
+
+An automated way of knowing if a test is available is to execute the
+'test/available' target from the test application directory.
+It executes without error if tests run by 'make test' are present.
+
+    make test/available