[LLDB] Update breakpoint-command.test to use string instead of number. (#69796)

lldb/test/Shell/Breakpoint/breakpoint-command.test adds a python
command, to be executed when a breakpoint hits, that writes out a
number. It then runs, hits the breakpoint and checks that the number is
present exactly once.

The problem is that on some systems the test can be run in a filepath
that happens to contain the number (e.g. auto-generated directory
names). The number is then detected multiple times and the test fails.

This patch fixes the issue by using a string instead, particularly a
string with spaces, which is very unlikely to be auto-generated by any
system.