[gdb/testsuite] Fix gdb.base/info-types-c++.exp with stressed cpu

When running test-case gdb.base/info-types-c++.exp in conjunction with:
...
$ stress -c $(($(cat /proc/cpuinfo | grep -c "^processor") + 1))
...
we get:
...
FAIL: gdb.base/info-types-c++.exp: info types (timeout)
...

Fix this by setting auto-solib-add to off.

Tested on x86_64-linux.