[gdb/testsuite] Fix gdb.base/readline.exp with stub-termcap

[gdb/testsuite] Fix gdb.base/readline.exp with stub-termcap

When doing a build which uses stub-termcap, we run into:
...
(gdb) set width 7
<b) FAIL: gdb.base/readline.exp: set width 7 (timeout)
...

Since readline can't detect very basic terminal support, it falls back on
horizontal scrolling.

Fix this by detecting the horizontal scrolling case, and skipping the
subsequent test.

Tested on x86_64-linux.

PR testsuite/30400
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30400