[circt-verilog] Fix failing CI check, NFC (#10292)

Fix CI failure introduced in: #10290.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>
Co-authored-by: Andrew Young <youngar17@gmail.com>

Author: seldridge

test/circt-verilog/xslang.sv

@@ -1,4 +1,5 @@
 // RUN: not circt-verilog %s -Xslang=--std=foo 2>&1 | FileCheck %s
 // RUN: circt-verilog %s -Xslang=--std=latest
 // REQUIRES: slang
+// UNSUPPORTED: valgrind
 // CHECK: invalid value for --std option: 'foo'