gdb.debuginfod/fetch_src_and_symbols.exp: fix when GDB is built with AddressSanitizer

This test fails for me, showing:

    ERROR: tcl error sourcing /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.debuginfod/fetch_src_and_symbols.exp.
    ERROR: This GDB was configured as follows:
       configure --host=x86_64-pc-linux-gnu --target=x86_64-pc-linux-gnu
                 --with-auto-load-dir=$debugdir:$datadir/auto-load
                 --with-auto-load-safe-path=$debugdir:$datadir/auto-load
    ... and much more ...

The problem is that TCL's exec throws an error as soon as the exec'ed
process outputs on stderr.  When GDB is built with ASan, it prints some
warnings about pre-existing signal handlers:

    warning: Found custom handler for signal 7 (Bus error) preinstalled.
    warning: Found custom handler for signal 8 (Floating point exception) preinstalled.
    warning: Found custom handler for signal 11 (Segmentation fault) preinstalled.

Pass --quiet to GDB to avoid these warnings.

Change-Id: I3751d89b9b1df646da19149d7cb86775e2d3e80f