[gdb/testsuite] Fix gdb.debuginfod/fetch_src_and_symbols.exp with nat…

…ive-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.

gdb/testsuite/gdb.debuginfod/fetch_src_and_symbols.exp

@@ -158,7 +158,9 @@ proc_with_prefix no_url { } {
     # Generate a core file and test that GDB cannot find the
     # executable.
     clean_restart ${binfile}2
-    gdb_test "start" "Temporary breakpoint.*"
+    if ![runto_main] {
+	return -1
+    }
     gdb_test "generate-core-file $::corefile" "Saved corefile $::corefile" \
 	"file [file tail $::corefile] gen"
     file rename -force ${binfile}2 $debugdir