Fix build breakage with --disable-tui

Fix build breakage with --disable-tui

An earlier patch introduced a unit test for tui_copy_source_line.
However if the TUI is not built (as is apparently the case on some of
the buildbot builders), then this will fail to link.

This patch fixes the problem.  Tested by rebuilding with the TUI
disabled.

gdb/ChangeLog
2019-12-02  Tom Tromey  <tom@tromey.com>

	* unittests/tui-selftests.c (run_tests): Make conditional.
	(_initialize_tui_selftest): Make conditional.

Change-Id: I964811c7635be24cf6c53920e74e920914503674