gdb/testsuite: fix invalid assumption about TUI src window

gdb/testsuite: fix invalid assumption about TUI src window

Fix a failing test introduced by this commit:

  commit e53b88b40ed38651b50f954dfe76066822094c15
  Date:   Wed Aug 13 15:29:38 2025 +0100

      gdb: fix forward/reverse search, when no lines are printed

The TUI test added in this commit assumed that the opening '{' of main
would be the first statement line (in DWARF terms), and so, would be
the initial focus of the TUI src window.

This is true for some targets (e.g. x86), but not
others (e.g. AArch64), and so gdb.tui/source-search.exp was seen
failing on at least some AArch64 targets.

Fix this by adding a 'list' command to the test, which forces the
initial window contents to be as needed for the rest of the test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33290
Approved-By: Tom de Vries <tdevries@suse.de>