Keep BASIC fixtures within the input line limit

Reflow the approved reference cases, shorten local fixture comments, and split the executable docs line so the 80-byte input buffer can read every source line.

Co-authored-by: andrew <andrew@aklabs.net>

tests/language/structures/missing_field.bas

@@ -1,5 +1,5 @@
-10 REM A field name is checked against a closed set the program declared, which
-20 REM is the one thing in this language whose valid spellings are written down.
+10 REM A field name is checked against the type's closed set, which
+20 REM is the one thing here whose valid spellings are written down.
 30 REM A misspelled *variable* is still silent -- see the last two lines.
 40 TYPE RECT
 50   W#