gdb/testsuite: resolve some duplicate test names in gdb.base

This commit resolves all the duplicate test names that I see in the
script:

  gdb.base/attach-pie-misread.exp

The duplicate names all come from a second call to
build_executable_own_libs, so in this commit I've places the second
call inside a with_test_prefix block.

While I was making this change I've also modified the value being
passed as the testname for the second build_executable_own_libs call.
Previously we used ${test}, however, I think this was likely a
mistake, the 'test' variable is setup for the previous test.  I
suspect that ${testfile} is a better choice - especially now we have a
testname prefix.

As the testname is only used (after various calls) from within
build_executable_from_specs should the build fail, I don't think this
change really makes much difference though.

There should be no change in what is tested after this commit.

gdb/testsuite/gdb.base/attach-pie-misread.exp

@@ -96,10 +96,12 @@ if {$result == 0} {
     fail $test
 }
 
-set prelink_args [build_executable_own_libs ${test}.exp $executable $srcfile \
-		      [list pie "additional_flags=-DGEN=\"$genfile\""]]
-if {$prelink_args == ""} {
-    return -1
+with_test_prefix "rebuild with DGEN defined" {
+    set prelink_args [build_executable_own_libs ${testfile}.exp $executable $srcfile \
+			  [list pie "additional_flags=-DGEN=\"$genfile\""]]
+    if {$prelink_args == ""} {
+	return -1
+    }
 }
 
 # x86_64 file has 25MB, no need to keep it.