Add testing for assertions inside unionized sections

test/link/section-union/assert.asm

@@ -0,0 +1,18 @@
+IF !DEF(SECOND)
+OFS = 42
+ELSE
+OFS = 69
+ENDC
+
+BASE = $C0DE
+
+SECTION UNION "assertions in unions", WRAM0
+IF DEF(SECOND)
+	assert @ != BASE, "Force failing the build" ; Force failure in RGBLINK, though
+ENDC
+	ds OFS
+	assert @ == BASE + OFS, "This assertion should not trigger"
+
+; Only make RGBASM aware of the section's location *after* it sees the assertion
+; This forces it to pass it to RGBLINK
+SECTION UNION "assertions in unions", WRAM0[BASE]