DSPSpy: Require user to specify a test_main label

This makes the point where execution starts more obvious compared to a start_of_test label at the end of the include, and allows putting other functions at the start of the file.  This change also modifies the existing tests to build with this change.

Source/DSPSpy/tests/andc_ls_test.ds

@@ -5,7 +5,7 @@ include "dsp_base.inc"
 ;    
 ;    3d80      andc'ls     $AC1.M : $AX0.L, $AC0.M
 
-
+test_main:
 	clr $ACC0
 	clr $ACC1
 	lri $AX0.L, #0x0000
@@ -14,7 +14,7 @@ include "dsp_base.inc"
 
 	lri $AC0.M, #0x0004
 	lri $AC1.M, #0x1234
-	andc'ls $AC1.M : $AX0.L, $AC0.M
+	cw 0x3d80 ; andc'ls $AC1.M : $AX0.L, $AC0.M
 
 	call send_back