qemu-arm: Fix tinytest test profile when updating set of dirs/files.

Updating a set must use `.update()` rather than `.add()`.

Also apply the same pattern to qemu-riscv to prevent the same issue when
directories/files are added to that port's `tests_profile.txt` file.

Signed-off-by: Damien George <damien@micropython.org>

ports/qemu-arm/tests_profile.txt

@@ -1,10 +1,10 @@
 # Port-specific test directories.
 
-test_dirs.add(("inlineasm", "qemu-arm"))
+test_dirs.update(("inlineasm", "ports/qemu-arm"))
 
 # Port-specific tests exclusion list.
 
-exclude_tests.add(
+exclude_tests.update(
     (
         # inline asm FP tests (require Cortex-M4)
         "inlineasm/asmfpaddsub.py",

ports/qemu-riscv/tests_profile.txt

@@ -1,3 +1,7 @@
+# Port-specific test directories.
+
+test_dirs.update(())
+
 # Port-specific tests exclusion list.
 
-exclude_tests = exclude_tests.union(())
+exclude_tests.update(())