[gdb/testsuite] Add gdb.base/eh_return.exp

In PR25350, an internal error was reported:
...
(gdb) break *eh2+0x7e
Breakpoint 1 at 0x13e2: file small.c, line 38.
(gdb) run
Starting program: a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".

Breakpoint 1, 0x00005555555553e2 in eh2 (
frame.c:558: internal-error: frame_id get_frame_id(frame_info*): \
  Assertion `stashed' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
...

The internal error does not reproduce after recent commit 547ce8f0
"[gdb/backtrace] Fix printing of fortran string args".

Add the corresponding test-case as regression test, given that the code is
rather atypical.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-08-16  Tom de Vries  <tdevries@suse.de>

	PR gdb/25350
	* gdb.base/eh_return.c: New test.
	* gdb.base/eh_return.exp: New file.