[gdb/testsuite] Update maint.exp for string cache

[gdb/testsuite] Update maint.exp for string cache

When running gdb.base/maint.exp, I see:
...
FAIL: gdb.base/maint.exp: maint print statistics
...

This is due to commit be1e3d3eab "Introduce objfile::intern", which replaces
the macro and filename caches with a string cache.

Update maint.exp accordingly.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-05  Tom de Vries  <tdevries@suse.de>

	* gdb.base/maint.exp: Update "main print statistics" expected output.