Skip tests that use cd for remote hosts

Skip tests that use cd for remote hosts

Several GDB tests change directory before compiling the test program
in order to test source file names that include directories.  This
doesn't work on a remote host because default_target_compile in
DejaGnu's target.exp copies each source file with
"[remote_download host $x]" which uses "[file tail $file] to strip
off the directory of each file.  If the source directory is remote
mounted on the host, this also leaves copied files in the source
directory.

A similar skip is already used in gdb.test/fullname.exp:

    # We rely on being able to copy things around.

    if { [is_remote host] } {
	untested "setting breakpoints by full path"
	return -1
    }

This patch causes three GDB tests that use "cd" to be skipped for a
remote host.  For gdb.base/fullpath-expand.exp this eliminates two
failures and prevents the test from leaving files fullpath-expand.c
and fullpath-expand-func.c in gdb/testsuite.  For
gdb.base/realname-expand.exp it eliminates two failures.  For
gdb.linespec/macro-relative.exp it prevents file macro-relative.c
from being left in gdb/testsuite/gdb.linespec/base/two.

gdb/testsuite/

	* gdb.base/fullpath-expand.exp: Skip for a remote host.
	* gdb.base/realname-expand.exp: Likewise.
	* gdb.linespec/macro-relative.exp: Likewise.