gdb/testsuite: squash duplicate test names in gdb.threads/*.exp

Resolve all of the duplicate test names in the gdb.threads/*.exp set
of tests (that I see).  Nothing very exciting here, mostly either
giving tests explicit testnames, or adding with_test_prefix.

The only interesting one is gdb.threads/execl.exp, I believe the
duplicate test name was caused by an actual duplicate test.  I've
remove the simpler form of the test.  I don't believe we've lost any
test coverage with this change.

gdb/testsuite/ChangeLog:

	* gdb.threads/execl.exp: Remove duplicate 'info threads' test.
	Make use of $gdb_test_name instead of creating a separate $test
	variable.
	* gdb.threads/print-threads.exp: Add a with_test_prefix instead of
	adding a '($name)' at the end of each test.  This also catches the
	one place where '($name)' was missing, and so caused a duplicate
	test name.
	* gdb.threads/queue-signal.exp: Give tests unique names to avoid
	duplicate test names based on the command being tested.
	* gdb.threads/signal-command-multiple-signals-pending.exp:
	Likewise.
	* lib/gdb.exp (gdb_compile_shlib_pthreads): Tweak test name to
	avoid duplicate testnames when a test script uses this proc and
	also gdb_compile_pthreads.
	* lib/prelink-support.exp (build_executable_own_libs): Use
	with_test_prefix to avoid duplicate test names when we call
	build_executable twice.