[gdb/testsuite] Limit verbose scope in gdb.base/break-interp.exp

[gdb/testsuite] Limit verbose scope in gdb.base/break-interp.exp

I'm running into the following failure (and 17 more like it) in
gdb.base/break-interp.exp:
...
 (gdb) bt^M
 #0  0x00007fde85a3b0c1 in __GI___nanosleep \
   (requested_time=requested_time@entry=0x7ffe5044ee70, \
   remaining=remaining@entry=0x7ffe5044ee70) at nanosleep.c:27^M
 #1  0x00007fde85a3affa in __sleep (seconds=0) at sleep.c:55^M
 #2  0x00007fde8606789c in libfunc (Reading in symbols for libc-start.c...^M
 action=0x7ffe5044fa12 "sleep") at gdb.base/break-interp-lib.c:41^M
 #3  0x0000000000400708 in main ()^M
 Reading in symbols for ../sysdeps/x86_64/start.S...^M
 (gdb) FAIL: gdb.base/break-interp.exp: LDprelinkNOdebugNO: \
   BINprelinkNOdebugNOpieNO: INNER: attach: attach main bt
...

The problem is that the test uses verbose mode to detect the "PIE (Position
Independent Executable) displacement" messages, but the verbose mode also
triggers "Reading in symbols for" messages, which may appear in the middle of
a backtrace (or not, depending on whether debug info is available).

[ In fact, the messages appear in the middle of a backtrace line, which is
PR25613. ]

Fix these FAILs by limiting the scope of verbose to the parts of the test that
need it.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.base/break-interp.exp: Limit verbose scope.