Fix some minor bugs in test suite command logging

Fix some minor bugs in test suite command logging

I noticed that the test suite command logging would create a file like
"gdb.cmd.-1".  I tracked this down to a substraction in
standard_output_file_with_gdb_instance.

Then, I saw that the .in file was not created for MI.  This is fixed
by adding a call to default_mi_gdb_start.

Finally, commands might not end up in the .in file in some cases.  For
me this happened because the test took a long time, so I got impatient
and killed it.  Flushing the file after each write seemed like a good
thing to do here.

gdb/testsuite/ChangeLog
2020-10-26  Tom Tromey  <tom@tromey.com>

	* lib/mi-support.exp (default_mi_gdb_start): Call
	gdb_stdin_log_init.
	* lib/gdb.exp (standard_output_file_with_gdb_instance): Don't
	subtract one from gdb_instances.
	(gdb_stdin_log_write): Flush in_file.