Fix unstable test names in gdb.arch/arc-tdesc-cpu.exp

Fix unstable test names in gdb.arch/arc-tdesc-cpu.exp

Currently if you diff testsuite/gdb.sum of two builds built from
different source trees you see this spurious hunk:

  -PASS: gdb.arch/arc-tdesc-cpu.exp: set tdesc filename /home/pedro/gdb1/src/gdb/testsuite/gdb.arch/arc-tdesc-cpu.xml
  +PASS: gdb.arch/arc-tdesc-cpu.exp: set tdesc filename /home/pedro/gdb2/src/gdb/testsuite/gdb.arch/arc-tdesc-cpu.xml

After this commit we'll show this instead in gdb.sum:
  PASS: gdb.arch/arc-tdesc-cpu.exp: set tdesc filename $srcdir/gdb.arch/arc-tdesc-cpu.xml

gdb/testsuite/ChangeLog:
2017-10-24  Pedro Alves  <palves@redhat.com>

	* gdb.arch/arc-tdesc-cpu.exp ('set tdesc filename'): Use gdb_test
	with explicit test name.