[gdb/testsuite] Fix gdb.base/maint.exp FAILs with -readnow

[gdb/testsuite] Fix gdb.base/maint.exp FAILs with -readnow

When running test-case gdb.base/maint.exp with target board readnow, we run
into:
...
FAIL: gdb.base/maint.exp: mt expand-symtabs
FAIL: gdb.base/maint.exp: maint print objfiles: psymtabs
FAIL: gdb.base/maint.exp: maint print psymbols -source
FAIL: gdb.base/maint.exp: maint print psymbols -pc
FAIL: gdb.base/maint.exp: maint info line-table with filename of symtab that \
  is not currently expanded
...

When using -readnow:
- there are no partial symtabs
- all symtabs are expanded at symbol load time
and these differences from normal behaviour cause the FAILs.

Update the tests for -readnow.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-27  Tom de Vries  <tdevries@suse.de>

	* gdb.base/maint.exp: Update for -readnow.