gdb/testsuite: update expected results in gdb.python/py-startup-opt.exp

gdb/testsuite: update expected results in gdb.python/py-startup-opt.exp

The test gdb.python/py-startup-opt.exp checks the behaviour of GDB's:

  set python dont-write-bytecode on

This flag (when on) stops Python creating .pyc files.  The test first
checks that .pyc files will be created, then turns this option on and
checks .pyc files will not be created.

However, if the user has PYTHONDONTWRITEBYTECODE set in their
environment then this will prevent Python from creating .pyc files, as
such the first test, that .pyc files are being created, currently
fails.

We could unset PYTHONDONTWRITEBYTECODE, however, until Python 3.8
there is no way to control where Python writes the .pyc files.  As the
GDB developer clearly doesn't want .pyc files created in their
file-system it feels wrong to silently unset this environment
variable.

My proposal then, is that we just spot when this environment variable
is set and adjust the expected results.  My hope is that across all
GDB developers some will be running with PYTHONDONTWRITEBYTECODE
unset, so this feature will be fully tested at least some of the time.

gdb/testsuite/ChangeLog:

	PR testsuite/27788
	* gdb.python/py-startup-opt.exp (test_python_settings): Change the
	expected results when environment variable PYTHONDONTWRITEBYTECODE
	is set.