gdb: Fix a few unstable test names

gdb: Fix a few unstable test names

Avoid putting unstable path names in test messages, in order to avoid
spurious testrun result diffs like:

 [....]
 -PASS: gdb.base/break-fun-addr.exp: /home/pedro/gdb/test-build1/gdb/testsuite/outputs/gdb.base/break-fun-addr/break-fun-addr1: break *main
 +PASS: gdb.base/break-fun-addr.exp: /home/pedro/gdb/test-build2/gdb/testsuite/outputs/gdb.base/break-fun-addr/break-fun-addr1: break *main
 [....]

gdb/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	* gdb.base/break-fun-addr.exp: Use $testfile1/$testfile2 for test
	prefix instead of $binfile1/$binfile2.
	* gdb.btrace/gcore.exp: Use "core" instead of unstable path name
	in test message.
	* gdb.python/py-completion.exp: Use "load python file" as test
	messages instead of unstable path names.