gdb/testsuite: Detect and warn about duplicate test names

Building on the previous commit, this patch detects when two tests
have the same test name and causes Dejagnu to print a new result type
'# of duplicate test names' in the result summary.  A line starting
with 'DUPLICATE: ' is also added to the gdb.sum and gdb.log files.

The DUPLICATE markers will be printed the second time a duplicate test
name is seen, and every time after that.  So you might see:

  PASS: gdb.base/sometest.exp: foo
  PASS: gdb.base/sometest.exp: bar
  PASS: gdb.base/sometest.exp: foo
  DUPLICATE: gdb.base/sometest.exp: foo
  PASS: gdb.base/sometest.exp: baz
  PASS: gdb.base/sometest.exp: foo
  DUPLICATE: gdb.base/sometest.exp: foo

However, the results will report a duplicate count of 1, indicating
that just one test name (foo) was duplicated.

Currently if the tests are run in parallel mode the new result type is
not merged into the combined summary file so users will need to run in
non-parallel mode to check this result.  Similarly, the 'DUPLICATE: '
markers will not be merged into the final gdb.sum file.  A later
commit will fix this.

gdb/testsuite/ChangeLog:

	* lib/check-test-names.exp (all_test_names): New module variable.
	(counts): Add 'duplicates' field.
	(_check_duplicates): New procedure.
	(check): Also check for duplicates.
	(do_log_summary): Print duplicates count.
	(do_reset_vars): Reset counter for duplicate test names, and
	discard all know test names.

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