std: add a couple of tests to valgrind module

lib/std/valgrind.zig

@@ -99,6 +99,10 @@ pub fn runningOnValgrind() usize {
     return doClientRequestExpr(0, .RunningOnValgrind, 0, 0, 0, 0, 0);
 }
 
+test "works whether running on valgrind or not" {
+    _ = runningOnValgrind();
+}
+
 /// Discard translation of code in the slice qzz.  Useful if you are debugging
 /// a JITter or some such, since it provides a way to make sure valgrind will
 /// retranslate the invalidated area.  Returns no value.
@@ -266,3 +270,8 @@ pub fn monitorCommand(command: [*]u8) bool {
 
 pub const memcheck = @import("valgrind/memcheck.zig");
 pub const callgrind = @import("valgrind/callgrind.zig");
+
+test "" {
+    _ = @import("valgrind/memcheck.zig");
+    _ = @import("valgrind/callgrind.zig");
+}

lib/std/valgrind/memcheck.zig

@@ -1,4 +1,5 @@
 const std = @import("../std.zig");
+const testing = std.testing;
 const valgrind = std.valgrind;
 
 pub const MemCheckClientRequest = extern enum {
@@ -142,6 +143,18 @@ pub fn countLeaks() CountResult {
     return res;
 }
 
+test "countLeaks" {
+    testing.expectEqual(
+        @as(CountResult, .{
+            .leaked = 0,
+            .dubious = 0,
+            .reachable = 0,
+            .suppressed = 0,
+        }),
+        countLeaks(),
+    );
+}
+
 pub fn countLeakBlocks() CountResult {
     var res: CountResult = .{
         .leaked = 0,
@@ -160,6 +173,18 @@ pub fn countLeakBlocks() CountResult {
     return res;
 }
 
+test "countLeakBlocks" {
+    testing.expectEqual(
+        @as(CountResult, .{
+            .leaked = 0,
+            .dubious = 0,
+            .reachable = 0,
+            .suppressed = 0,
+        }),
+        countLeakBlocks(),
+    );
+}
+
 /// Get the validity data for addresses zza and copy it
 /// into the provided zzvbits array.  Return values:
 ///    0   if not running on valgrind