testsuite: Disable backslash_in_multi_line_command_test for old DejaGnus

testsuite: Disable backslash_in_multi_line_command_test for old DejaGnus

I noticed that backslash_in_multi_line_command_test in
gdb.base/commands.exp failed on our RHEL6 servers.  I traced it to the
old version of DejaGnu (1.4.4).  I have found that instead of receiving
the expected:

  "print \\\nargc\n"

gdb received:

  "print  argc\n"

thus breaking the test and its purpose.  Versionof DejaGnu < 1.5 mess
up sending "\\\n", it somehow gets replaced with a space.  I found that
the following commit in DejaGnu fixed the issue:

http://git.savannah.gnu.org/cgit/dejagnu.git/commit/lib/remote.exp?id=3f39294f5cd6802858838d3bcc0ccce847ae17f2

Even though the commit is almost 10 years old, the following release of
DejaGnu was only in 2013, which is why we still have systems with the
old code.

If the DejaGnu version is < 1.5, we just skip the test.

gdb/testsuite/ChangeLog:

	* gdb.base/commands.exp (backslash_in_multi_line_command_test):
	Skip for versions of DejaGnu < 1.5.