natives/strings: Skip TestBuilderGrow.

The test has started relying on testing.AllocsPerRun and
runtime.ReadMemStats in Go 1.11, which are not supported
by GopherJS. Skip the test, just like TestBuilderAllocs.

Fixes:

	$ gopherjs test --short strings
	--- FAIL: TestBuilderGrow (0.24s)
	    test.992502099:29524: growLen=100: got 0 allocs during Write; want 1
	    test.992502099:29524: growLen=1000: got 0 allocs during Write; want 1
	    test.992502099:29524: growLen=10000: got 0 allocs during Write; want 1
	    test.992502099:29524: growLen=100000: got 0 allocs during Write; want 1
	FAIL
	FAIL  strings     1.501s

compiler/natives/src/strings/strings_test.go

@@ -7,3 +7,7 @@ import "testing"
 func TestBuilderAllocs(t *testing.T) {
 	t.Skip("runtime.ReadMemStats, testing.AllocsPerRun not supported in GopherJS")
 }
+
+func TestBuilderGrow(t *testing.T) {
+	t.Skip("runtime.ReadMemStats, testing.AllocsPerRun not supported in GopherJS")
+}