gdb/testsuite: don't include port numbers in test names

gdb/testsuite: don't include port numbers in test names

The gdb.python/py-cmd-prompt.exp script includes a test that has a
gdbserver port number within a test name.  As port numbers can change
from one test run to the next (depending on what else is running on
the machine at the time), this can make it hard to compare test
results between runs.

Give the test a specific name to avoid including the port number.

There is no change in what is tested after this commit.