Refactor the tests that allow some round errors to use custom Matcher (#134)

to simplify the test code.

flexbox/src/androidTest/java/com/google/android/flexbox/test/IsEqualAllowingError.java

+/*
+ * Copyright 2016 Google Inc. All rights reserved.
+ *
+ * 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.
+ */
+
+package com.google.android.flexbox.test;
+
+import org.hamcrest.BaseMatcher;
+import org.hamcrest.Description;
+import org.hamcrest.Factory;
+import org.hamcrest.Matcher;
+
+import java.util.Locale;
+
+/**
+ * Custom {@link BaseMatcher} that expects {@link Number} value allowing some errors to allow
+ * such as rounding errors.
+ */
+class IsEqualAllowingError<T extends Number> extends BaseMatcher<T> {
+
+    private Number expected;
+
+    private Integer errorAllowed;
+
+    private IsEqualAllowingError(Number expected) {
+        this(expected, 1);
+    }
+
+    private IsEqualAllowingError(Number expected, int errorAllowed) {
+        this.expected = expected;
+        this.errorAllowed = errorAllowed;
+    }
+
+    @Override
+    public boolean matches(Object item) {
+        if (!(item instanceof Number)) {
+            return false;
+        }
+        Number other = (Number) item;
+        return expected.intValue() - errorAllowed <= other.intValue() &&
+                other.intValue() <= expected.intValue() + errorAllowed;
+    }
+
+    @Override
+    public void describeTo(Description description) {
+        description.appendText(
+                String.format(Locale.US, "expected value is <%s> allowing error of <%s>.", expected,
+                        errorAllowed));
+    }
+
+    @Factory
+    static <T extends Number> Matcher<T> isEqualAllowingError(T expected) {
+        return new IsEqualAllowingError<>(expected);
+    }
+
+    @Factory
+    static <T extends Number> Matcher<T> isEqualAllowingError(T expected, int errorAllowed) {
+        return new IsEqualAllowingError<>(expected, errorAllowed);
+    }
+}
\ No newline at end of file