Add tests for purging symbols

test/asm/purge-refs.asm

@@ -0,0 +1,13 @@
+
+SECTION "floating purging", ROM0
+
+Floating:
+	db Floating
+	PURGE Floating ; This used to cause a use-after-free
+	; To increase the chances of the data being re-allocated, it's not last
+
+SECTION "fixed purging", ROM0[0]
+
+Fixed:
+	db Fixed
+	PURGE Fixed ; Can work, it's a constant

test/asm/purge-refs.err

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+ERROR: purge-refs.asm(6):
+    Symbol "Floating" is referenced and thus cannot be purged
+error: Assembly aborted (1 errors)!

test/asm/purge.asm

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+
+SECTION "purging", ROM0
+
+Label:
+	PURGE Label
+Label:
+
+	db Referenced
+	PURGE Referenced ; Should fail
+
+Exported::
+	PURGE Exported
+Exported::

test/asm/purge.err

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+ERROR: purge.asm(9):
+    Symbol "Referenced" is referenced and thus cannot be purged
+error: Assembly aborted (1 errors)!