add tests for racket/port

Also, fix some doc typos.

pkgs/racket-doc/scribblings/reference/string-input.scrbl

@@ -235,9 +235,9 @@ arguments. The four arguments correspond to the location of the
 special value within the port, as described in
 @secref["customport"]. If the procedure is called more than once
 with valid arguments, the @exnraise[exn:fail:contract]. If
-@racket[read-bytes-avail] returns a special-producing procedure, then
+@racket[read-bytes-avail!] returns a special-producing procedure, then
 it does not place characters in @racket[bstr]. Similarly,
-@racket[read-bytes-avail] places only as many bytes into @racket[bstr]
+@racket[read-bytes-avail!] places only as many bytes into @racket[bstr]
 as are available before a special value in the port's stream.}
 
 @defproc[(read-bytes-avail!* [bstr bytes?]
@@ -351,7 +351,7 @@ case that @racket[progress] becomes ready before bytes are peeked.}
 Like @racket[read-bytes-avail!*], but for @tech{peek}ing, and with
 @racket[skip-bytes-amt] and @racket[progress] arguments like
 @racket[peek-bytes-avail!]. Since this procedure never blocks, it may
-return before even @racket[skip-amt] bytes are available from the
+return before even @racket[skip-bytes-amt] bytes are available from the
 port.}
 
 @defproc[(peek-bytes-avail!/enable-break [bstr (and/c bytes? (not/c immutable?))]

pkgs/racket-test-core/tests/racket/portlib.rktl

@@ -1201,4 +1201,43 @@
 
 ;; --------------------------------------------------
 
+(let ()
+  ;; Check `special-filter-input-port`
+  (define-values (i o) (make-pipe-with-specials))
+  (define fi (special-filter-input-port
+              i
+              (lambda (proc bstr)
+                (bytes-set! bstr 0 (char->integer #\z))
+                1)))
+  (write-bytes #"abc" o)
+  (test #"abc" read-bytes 3 fi)
+  (write-special 'hello o)
+  (test #"z" read-bytes 1 fi)
+  (write-bytes #"ab" o)
+  (write-bytes #"c" o)
+  (write-special 'ok o)
+  (write-special 'bye o)
+  (test #"abczz" peek-bytes 5 0 fi)
+  (test #"abczz" peek-bytes 5 0 fi)
+  (test #"abcz" read-bytes 4 fi)
+  (define bstr (make-bytes 5))
+  (test 1 peek-bytes-avail! bstr 0 #f fi)
+  (test #"z" subbytes bstr 0 1))
+
+(let ()
+  ;; Check `special-filter-input-port` with `peeking-input-port`
+  (define-values (i o) (make-pipe-with-specials))
+  (define fi (special-filter-input-port
+              i
+              (lambda (proc bstr)
+                (bytes-set! bstr 0 (char->integer #\z))
+                1)))
+  (define pi (peeking-input-port fi))
+  (write-bytes #"abc" o)
+  (write-special 'hello o)
+  (write-special 'again o)
+  (test #"abczz" peek-bytes 5 0 pi))
+
+;; --------------------------------------------------
+
 (report-errs)