[PATCH, testsuite] Fix subst.exp failure when GDB is configured with --with-relocated-sources

[Luis Machado]

Hi,

The gdb.base/subst.exp test assumes GDB starts with no default 
substitution rules, which is not true if GDB has been configured with 
--with-relocated-sources. In that case we start with a default rule, 
causing the first test (checking that there are no rules) to fail.

This patch clears any default substitution rules that may exist prior to 
doing all the other tests.

OK?
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