Skip tests on completion and readline when readline lib isn't used

The completion feature and other features on readline depend on the
readline library.  However, readline library is not always used, for
example, running testsuite like

  make check RUNTESTFLAGS="--host_board=local-remote-host"

the input stream is not a tty, and GDB doesn't use readline library
as a result.

This patch is to skip tests on completion and readline if
'show editing' is off, which means readline isn't used.  Note that
some tests in gdb.base/completion.exp test command complete, which
isn't related to readline, so these tests aren't affected by readline
library.  This patch also moves these tests up, run them
unconditionally, and run the rest if readline library is used.

gdb/testsuite:

2014-03-26  Yao Qi  <yao@codesourcery.com>

	* lib/gdb.exp (readline_is_used): New proc.
	* gdb.base/completion.exp: Move tests on command complete up.
	Skip the rest of tests if readline is not used.
	* gdb.ada/complete.exp: Skp the test if readline is not
	used.
	* gdb.base/filesym.exp: Likewise.
	* gdb.base/macscp.exp: Likewise.
	* gdb.base/readline-ask.exp: Likewise.
	* gdb.base/readline.exp: Likewise.
	* gdb.python/py-cmd.exp: Likewise.
	* gdb.trace/tfile.exp: Likewise.