[gdb/testsuite] Fix gdb.debuginfod/fetch_src_and_symbols.exp with native-gdbserver

[gdb/testsuite] Fix gdb.debuginfod/fetch_src_and_symbols.exp with native-gdbserver

When running test-case gdb.debuginfod/fetch_src_and_symbols.exp with target
board native-gdbserver, I get:
...
Running gdb.debuginfod/fetch_src_and_symbols.exp ...
ERROR: tcl error sourcing gdb.debuginfod/fetch_src_and_symbols.exp.
ERROR: gdbserver does not support start without extended-remote
    while executing
"error "gdbserver does not support $command without extended-remote""
    (procedure "gdb_test_multiple" line 51)
    invoked from within
"gdb_test_multiple $command $message {*}$opts $user_code"
    (procedure "gdb_test" line 56)
    invoked from within
"gdb_test "start" "Temporary breakpoint.*""
...

Fix this by replacing gdb_test "start" with runto_main.

Tested on x86_64-linux.