fix os.date formatting with %x

[Andriamanitra]

Changes proposed in this pull request:

• change format code %x to mean %m/%d/%y to match Lua (https://www.lua.org/pil/22.1.html) (before this PR it was %H/%M/%S I assume by mistake)
• add a test for date formatting

While writing the test I also noticed a few other date formatting codes that don't exactly match what Lua does:

• %p is "PM" ("pm" in Lua)
• %c is "02 Jan 06 15:04 MST" ("Mon Jan 2 15:04:05 2006" in Lua)

I decided to not include them in the test. I also left formatting codes that are not documented on lua.org untested.

The issue was first reported in micro: micro-editor/micro#4067

[Andriamanitra]

There is also #465 which is a bigger change that addresses the same issue, but it has been in limbo for a while. I will close this PR if that one is preferable.