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#

tests/language/structures/parameters.bas

@@ -17,7 +17,7 @@
 170 RETURN B@.W#
 180 PRINT WIDEN(A@)
 190 PRINT A@.W#
-200 REM To change one on purpose, pass a pointer. Assignment copies a pointer's
+200 REM To change one, pass a pointer. Assignment copies the pointer.
 210 REM reference, so the callee is looking at the caller's own record.
 220 DIM Q@ AS PTR TO CRATE
 230 POINT Q@ AT A@
@@ -37,7 +37,7 @@
 370 N2@.COUNT# = 20
 380 POINT N1@.TAIL@ AT N2@
 390 DEF TOTAL(P@ AS PTR TO NODE)
-395 REM A pointer is true when it points at something, which is how a walk knows
+395 REM A pointer is true when it points at something, so a walk knows
 396 REM where the list ends. NOT is the bitwise operator here, so the test is
 397 REM written the positive way round.
 400 IF P@->TAIL@ THEN RETURN P@->COUNT# + TOTAL(P@->TAIL@)

tests/language/structures/records.bas

@@ -1,4 +1,4 @@
-10 REM A TYPE declares its fields; each takes its own type from its own suffix,
+10 REM A TYPE declares fields; each takes its type from its own suffix,
 20 REM which is the same rule every other name in this language follows.
 30 TYPE COORD
 40   X#

tests/language/structures/reserved_names.bas

@@ -1,6 +1,6 @@
 10 REM A type name and a field name are bare words, and so is every verb, so
 20 REM they share a namespace whether we like it or not. Both are refused with
-30 REM the same rule the scanner already applies to variable names -- and a type
+30 REM the same rule the scanner applies to variable names -- and a type
 40 REM name is refused by the prescan, which can say so plainly rather than
 50 REM leaving the parser to report "Expected expression or literal".
 60 TYPE POINT