gdb/testsuite: set ASAN_OPTIONS=detect_leaks=0 while running tests

gdb/testsuite: set ASAN_OPTIONS=detect_leaks=0 while running tests

We see some additional failures when running the testsuite against a GDB
compiled with ASan, compared to a GDB compiled without ASan.  Some of
them are caused by the memory leak report shown by the GDB process when
it exits, and the fact that it makes it exit with a non-zero exit code.

I generally try to remember to set ASAN_OPTIONS=detect_leaks=0 in my
environment when running the tests, but I don't always do it.  I think
it would be nice if the testsuite did it.  I don't see any use to have
leak detection when running the tests.  That is, unless we ever have a
test that ensures GDB doesn't leak memory, which isn't going to happen
any time soon.

Here are some tests I found that were affected by this:

    gdb.base/batch-exit-status.exp
    gdb.base/many-headers.exp
    gdb.base/quit.exp
    gdb.base/with-mf.exp
    gdb.dwarf2/gdb-add-index.exp
    gdb.dwarf2/gdb-add-index-symlink.exp
    gdb.dwarf2/imported-unit-runto-main.exp

Change-Id: I784c7df8a13979eb96587f735c1d33ba2cc6e0ca