Fix unstable test names in gdb.gdb/unittest.exp

Fix unstable test names in gdb.gdb/unittest.exp

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

  -PASS: gdb.gdb/unittest.exp: maintenance check xml-descriptions /home/pedro/gdb1/src/gdb/testsuite/../features
  +PASS: gdb.gdb/unittest.exp: maintenance check xml-descriptions /home/pedro/gdb2/src/gdb/testsuite/../features

After this commit we'll show instead:

  PASS: gdb.gdb/unittest.exp: maintenance check xml-descriptions ${srcdir}/../features

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

	* gdb.gdb/unittest.exp ('maintenance check xml-descriptions'): Use
	custom test name.