test_gdb: uncomment and fix testBreakOnReadBreakpoint()

a46018a9 commented out the definition of
testBreakOnReadBreakpoint() because of a GDB bug that was fixed at some
point before GDB 14.2 release.

The next commit a46018a9 implemented
another test function testBreakOnReadBreakpoint2() to work around the
GDB bug. This second test function is different and therefore also
useful. Explain its historical purpose in a comment.