[gdb/testsuite] Fix gdb.rust/traits.exp with -readnow

With test-case gdb.rust/traits.exp and target board readnow we get:
...
FAIL: gdb.rust/traits.exp: print *td
FAIL: gdb.rust/traits.exp: print *tu
...

Mark these FAILs as KFAILs.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-28  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (readnow): Handle arg.
	* gdb.rust/traits.exp: Add KFAILs for -readnow.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-10-28  Tom de Vries  <tdevries@suse.de>
+
+	* lib/gdb.exp (readnow): Handle arg.
+	* gdb.rust/traits.exp: Add KFAILs for -readnow.
+
 2020-10-28  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.base/relocate.exp: Update regexp for -readnow.

gdb/testsuite/gdb.rust/traits.exp

@@ -43,5 +43,13 @@ if {![runto ${srcfile}:$line]} {
     return -1
 }
 
+set readnow_p [readnow $binfile]
+
+if { $readnow_p } {
+    setup_kfail "gdb/26799" *-*-*
+}
 gdb_test "print *td" " = 23.5"
+if { $readnow_p } {
+    setup_kfail "gdb/26799" *-*-*
+}
 gdb_test "print *tu" " = 23"

gdb/testsuite/lib/gdb.exp

@@ -7318,8 +7318,13 @@ gdb_caching_proc supports_fcf_protection {
 
 # Return 1 if symbols were read in using -readnow.  Otherwise, return 0.
 
-proc readnow { } {
-    set cmd "maint print objfiles"
+proc readnow { args } {
+    if { [llength $args] == 1 } {
+	set re [lindex $args 0]
+    } else {
+	set re ""
+    }
+    set cmd "maint print objfiles $re"
     gdb_test_multiple $cmd "" {
 	-re -wrap "\r\n.gdb_index: faked for \"readnow\"\r\n.*" {
 	    return 1