Sniff umask behavior

This commit attempts to detect "anomalous" process.umask behavior.
In certain environments on Windows, process.umask does not exhibit
expected behavior; e.g., failing to set a new mask. Here, we add a
script to try and detect deviant behavior and skip tests if this
behavior is detected. This allows us to avoid failing builds on,
e.g., AppVeyor.

- https://ci.appveyor.com/project/kgryte/stdlib/build/job/7o6qgf25sovv9bea

Author: kgryte

lib/node_modules/@stdlib/process/umask/test/fixtures/check_umask.js

@@ -0,0 +1,73 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2018 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+'use strict';
+
+// MAIN //
+
+/**
+* Checks whether the built-in `process.umask` exhibits expected behavior.
+*
+* ## Notes
+*
+* -   In certain environments on Windows, `process.umask` does not seem to behave as expected (e.g., a mask cannot be set). This function is indicated to "sniff" this behavior.
+*
+* @private
+* @returns {boolean} boolean indicating whether `process.umask` exhibits expected behavior
+*/
+function check() {
+	var MASK;
+	var mask;
+	var bool;
+	var old;
+	var i;
+
+	MASK = process.umask();
+	bool = true;
+
+	for ( i = 100; i < 200; i++ ) {
+		old = process.umask();
+
+		// Should return an integer-valued number:
+		if ( typeof old !== 'number' ) {
+			bool = false;
+			break;
+		}
+
+		// Attempt to update the mask:
+		mask = process.umask( i );
+
+		// Should have returned the previous mask:
+		if ( mask !== old ) {
+			bool = false;
+			break;
+		}
+		// Should return the current mask:
+		if ( process.umask() !== i ) {
+			bool = false;
+			break;
+		}
+	}
+	process.umask( MASK ); // restore
+	return bool;
+}
+
+
+// EXPORTS //
+
+module.exports = check;

lib/node_modules/@stdlib/process/umask/test/test.js

@@ -22,6 +22,7 @@
 
 var tape = require( 'tape' );
 var IS_BROWSER = require( '@stdlib/assert/is-browser' );
+var check = require( './fixtures/check_umask.js' );
 var umask = require( './../lib' );
 
 
@@ -36,7 +37,7 @@ var INT_TO_SYMBOLIC = int2symbolic();
 var SYMBOLIC_TO_INT = symbolic2int();
 var MASK = umask();
 var opts = {
-	'skip': IS_BROWSER
+	'skip': IS_BROWSER || check()
 };