[gdb/testsuite] Fix gdb.base/include-main.exp with -readnow

With test-case gdb.base/include-main.exp and target board readnow, I run into:
...
FAIL: gdb.base/include-main.exp: maint info symtab
...

The corresponding check in gdb.base/include-main.exp:
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gdb_test_no_output "maint info symtab"
...
checks that no CU was expanded, while -readnow ensures that all CUs are
expanded.

Fix this by skipping the check with -readnow.

Tested on x86_64-linux, with native and target board readnow.