Fix unstable test names in gdb.python/py-objfile.exp

Currently, if you diff testsuite/gdb.sum of different builds you see
this spurious hunk:

  -PASS: gdb.python/py-objfile.exp: get python valueof "sep_objfile.build_id" (6a0bfcab663f9810ccff33c756afdebb940037d4)
  +PASS: gdb.python/py-objfile.exp: get python valueof "sep_objfile.build_id" (1f5531c657c57777b05fc95baa0025fd1d115c3b)

Fix this by syncing get_python_valueof with get_integer_valueof, which
stopped outputting the value in commit 2f20e312aad6
("get_integer_valueof: Don't output value in test name").

After this commit we'll show:

  PASS: gdb.python/py-objfile.exp: get python valueof "sep_objfile.build_id"

As the comment explicitly says get_python_valueof is modeled on
get_integer_valueof, I went ahead and also added the optional 'test'
parameter while at it.

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

	* lib/gdb-python.exp (get_python_valueof): Add 'test' optional
	parameter and handle it.  Don't output read value in test name.

1 files changed, 10 insertions, 6 deletions

diff --git a/gdb/testsuite/lib/gdb-python.exp b/gdb/testsuite/lib/gdb-python.exp
index 1fef27ed67..c0c0d68259 100644
--- a/gdb/testsuite/lib/gdb-python.exp
+++ b/gdb/testsuite/lib/gdb-python.exp
@@ -46,19 +46,23 @@ proc gdb_py_test_multiple { name args } {
     return 0
 }
 
-# Return the result of python expression EXPR.
-# DEFAULT is returned if there's an error.
-# This is modelled after get_integer_valueof.
+# Return the result of python expression EXPR.  DEFAULT is returned if
+# there's an error.  TEST is the test message to use.  It can be
+# omitted, in which case a test message is built from EXP.  This is
+# modeled after get_integer_valueof.
 
-proc get_python_valueof { exp default } {
+proc get_python_valueof { exp default {test ""} } {
     global gdb_prompt
 
-    set test "get python valueof \"${exp}\""
+    if {$test == ""} {
+	set test "get python valueof \"${exp}\""
+    }
+
     set val ${default}
     gdb_test_multiple "python print (\"valueof: %s\" % (${exp}))" "$test" {
 	-re "valueof: (\[^\r\n\]*)\[\r\n\]*$gdb_prompt $" {
 	    set val $expect_out(1,string)
-	    pass "$test ($val)"
+	    pass "$test"
 	}
 	timeout {
 	    fail "$test (timeout)"