New testcase for PR tui/25126 (staled source cache)

New testcase for PR tui/25126 (staled source cache)

I'm dealing with a Fedora GDB bug that is related to PR tui/25126, and
I thought I'd write a specific testcase for it because I couldn't find
one.

The idea is to get the simple reproducer from the bug and tweak the
testcase around it.  This one was a bit hard because, since we need to
modify the source file and recompile it, it involved a bit of TCL-foo
to do things.  Also for this reason, I'm only enabling the test for
native boards.

I tested this with an upstream GDB and made sure everything is
passing.  I also tested with a faulty GDB and made sure the test
failed.

gdb/testsuite/ChangeLog:
2020-02-11  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR tui/25126
	https://bugzilla.redhat.com/show_bug.cgi?id=1784210
	* gdb.base/cached-source-file.c: New file.
	* gdb.base/cached-source-file.exp: New file.

Change-Id: Ib1b074342ebe8613c6d1dfde631691ebdf6d81c6