gentest for Java and C#

Summary:
- Revise scripts to generate Java and C# unittests using the same HTML fixtures as well as C for code coverage.
- Add wrap_column test workaround in gentest.js.
- Add checkDefaultValues for sanity check of the CSSLayout default values by test-template.html
- Add `align-content: flex-start;` in default div to align with CSSLayout default

    $ cd csharp/gentest
    $ ruby gentest.rb

- macOS example for C#

    $ cd csharp/tests/Facebook.CSSLayout
    $ clang -DCSS_ASSERT_FAIL_ENABLED -Wall -Wextra -dynamiclib -o libCSSLayout.dylib -g -I../../.. ../../../CSSLayout/*.c ../../CSSLayout/CSSInterop.cpp
    $ mcs -debug -t:library -r:nunit.framework.dll -out:CSSLayoutTest.dll *.cs ../../../csharp/Facebook.CSSLayout/*cs
    $ mono64 --debug nunit-console.exe CSSLayoutTest.dll

Reviewed By: emilsjolander

Differential Revision: D4053777

fbshipit-source-id: 84450208015e65baf604987bd56c6a268598b545

gentest/test-template.html

@@ -4,6 +4,9 @@
     <meta charset="UTF-8">
     <title>test page</title>
     <script src="gentest.js"></script>
+    <script src="gentest-cpp.js"></script>
+    <script src="gentest-java.js"></script>
+    <script src="gentest-cs.js"></script>
 
     <style>
       body {
@@ -23,6 +26,7 @@
         display: flex;
         flex-direction: column;
         align-items: stretch;
+        align-content: flex-start;
         justify-content: flex-start;
         flex-shrink: 0;
       }