runtime: re-add sanity check for GCCPUFraction

This no longer appears to be reproducible on windows/386. Try putting
it back and we'll see if the builders still don't like it.

Fixes #19319.

Change-Id: Ia47b034e18d0a5a1951125c00542b021aacd5e8d
Reviewed-on: https://go-review.googlesource.com/47936
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/malloc_test.go

@@ -46,9 +46,6 @@ func TestMemStats(t *testing.T) {
 	}
 	// Of the uint fields, HeapReleased, HeapIdle can be 0.
 	// PauseTotalNs can be 0 if timer resolution is poor.
-	//
-	// TODO: Test that GCCPUFraction is <= 0.99. This currently
-	// fails on windows/386. (Issue #19319)
 	fields := map[string][]func(interface{}) error{
 		"Alloc": {nz, le(1e10)}, "TotalAlloc": {nz, le(1e11)}, "Sys": {nz, le(1e10)},
 		"Lookups": {nz, le(1e10)}, "Mallocs": {nz, le(1e10)}, "Frees": {nz, le(1e10)},
@@ -61,7 +58,7 @@ func TestMemStats(t *testing.T) {
 		"NextGC": {nz, le(1e10)}, "LastGC": {nz},
 		"PauseTotalNs": {le(1e11)}, "PauseNs": nil, "PauseEnd": nil,
 		"NumGC": {nz, le(1e9)}, "NumForcedGC": {nz, le(1e9)},
-		"GCCPUFraction": nil, "EnableGC": {eq(true)}, "DebugGC": {eq(false)},
+		"GCCPUFraction": {le(0.99)}, "EnableGC": {eq(true)}, "DebugGC": {eq(false)},
 		"BySize": nil,
 	}