gbasm: add support for generating source input

By adding an "init" function per test, it is easier to
generate code that would be laborious to write by hand.

src/gbasm/tests/test.c

@@ -36,6 +36,10 @@ static void run_test(const void *test_data)
 	int rc;
 	size_t len;
 
+	if (test->init != NULL) {
+		test->init();
+	}
+
 	src = strdup(test->asm_source);
 	
 	memset(program, 0, sizeof(program));

src/gbasm/tests/test.h

@@ -4,10 +4,11 @@
 #include <stdint.h>
 
 struct gbasm_test {
+	void (*init)(void);
 	const char *name;
 	const char *asm_source;
 	const uint8_t *expected_output;
-	const int expected_output_len;
+	int expected_output_len;
 };
 
 struct gbasm_tests {