[slang] Disable valgrind for failing test

Exclude a recently added slang-related test from valgrind.  Slang v3 has
some problems with jumps depending on uninitialized values.  This works
around that.

Fixes #7958.

h/t @fabianschuiki for the fix.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Author: seldridge

test/Conversion/ImportVerilog/hierarchical-names.sv

@@ -2,6 +2,9 @@
 // RUN: circt-verilog --ir-moore %s
 // REQUIRES: slang
 
+// Internal issue in Slang v3 about jump depending on uninitialised value.
+// UNSUPPORTED: valgrind
+
 // CHECK-LABEL: moore.module @Foo()
 module Foo;
   int r;