[patch, testsuite] check for readline support in gdb.base/history-duplicates.exp

[Sandra Loosemore]

The testcase gdb.base/history-duplicates.exp checks the interaction 
between "set history remove-duplicates" and scrolling back through 
command history with ^P, thus it only makes sense if readline support is 
present.  I've copied the test that currently appears in readline.exp 
and other places.  OK to commit?

-Sandra

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