Fix all PATH problems in testsuite/gdb.perf/

Fix all PATH problems in testsuite/gdb.perf/

Currently "make check-perf" triggers ~40 PATH messages in gdb.sum:

  ...
  PATH: gdb.perf/backtrace.exp: python sys.path.insert(0, os.path.abspath("/home/pedro/rocm/gdb/build/gdb/../../src/gdb/testsuite/gdb.perf/lib"))
  PATH: gdb.perf/backtrace.exp: python exec (open ('/home/pedro/rocm/gdb/build/gdb/testsuite/outputs/gdb.perf/backtrace/backtrace.py').read ())
  ...

This commit fixes them.  E.g. before/after gdb.sum diff:

 -PASS: gdb.perf/backtrace.exp: python import os, sys
 -PASS: gdb.perf/backtrace.exp: python sys.path.insert(0, os.path.abspath("/home/pedro/rocm/gdb/build-master/gdb/../../src/gdb/testsuite/gdb.perf/lib"))
 -PATH: gdb.perf/backtrace.exp: python sys.path.insert(0, os.path.abspath("/home/pedro/rocm/gdb/build-master/gdb/../../src/gdb/testsuite/gdb.perf/lib"))
 -PASS: gdb.perf/backtrace.exp: python exec (open ('/home/pedro/rocm/gdb/build-master/gdb/testsuite/outputs/gdb.perf/backtrace/backtrace.py').read ())
 -PATH: gdb.perf/backtrace.exp: python exec (open ('/home/pedro/rocm/gdb/build-master/gdb/testsuite/outputs/gdb.perf/backtrace/backtrace.py').read ())
 +PASS: gdb.perf/backtrace.exp: setup perftest: python import os, sys
 +PASS: gdb.perf/backtrace.exp: setup perftest: python sys.path.insert(0, os.path.abspath("${srcdir}/gdb.perf/lib"))
 +PASS: gdb.perf/backtrace.exp: setup perftest: python exec (open ('${srcdir}/gdb.perf/backtrace.py').read ())

gdb/testsuite/
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* lib/perftest.exp (PerfTest::_setup_perftest): Use
	with_test_prefix.  Add explicit test names to python invocations,
	with "$srcdir" not expanded.

Change-Id: I50a31b04b7abdea754139509e4a34ae9263118a4