[gdb/testsuite] Fix timeout in gdb.ada/call_pn.exp

[gdb/testsuite] Fix timeout in gdb.ada/call_pn.exp

With test-case gdb.ada/call_pn.exp and glibc debug info installed, I ran into
this timeout:
...
(gdb) maint expand-symtabs^M
FAIL: gdb.ada/call_pn.exp: maint expand-symtabs (timeout)
...

The timeout was related to running the cpu at base frequency of 400Mhz instead
of boost frequency of 3.5Ghz (efficiency core) or 4.7Ghz (performance core).

But when investigating the test-case I realized that the maint expand-symtabs
could be limited to the source files, so use that to speed up the test-case.

Tested on x86_64-linux.

Co-Authored-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/32177
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32177