[gdb/testsuite] Handle USE_TUI in gdb.tui/corefile-run.exp

[gdb/testsuite] Handle USE_TUI in gdb.tui/corefile-run.exp

Once in a while I find myself rewriting a TUI test-case into a non-TUI
test-case, to better understand whether the problem I'm looking at is
related to the TUI or not.

I've got the impression that I've done this sufficiently often that it's worth
committing the non-TUI version, so having just written a non-TUI version of
gdb.tui/corefile-run.exp, let's commit it.

The non-TUI version can be enabled by doing:
...
$ make check "RUNTESTFLAGS=gdb.tui/corefile-run.exp USE_TUI=0"
...

Also remove hard-coding of a source line number.

Tested on x86_64-linux.