[testsuite] Skip gdb.trace/range-stepping.exp test case if not supported

[testsuite] Skip gdb.trace/range-stepping.exp test case if not supported

Tracepoints and range stepping are independent features.  This patch
skips the gdb.trace/range-stepping.exp test case if the target does not
support range stepping.

gdb/testsuite/ChangeLog:

	* gdb.base/range-stepping.exp (gdb_range_stepping_enabled):
	Move it to ...
	* lib/range-stepping-support.exp (gdb_range_stepping_enabled):
	... here.
	* gdb.trace/range-stepping.exp: Check that the target supports
	range stepping.