Add test standards and test templates to CONTRIBUTING.md

[AlpinYukseloglu]

Closes: #1954

What is the purpose of the change

This PR introduces testing standards and templates to our CONTRIBUTING.md to align expectations on testing while we continue to refactor and clean up our old tests.

Brief Changelog

Add section for writing tests to CONTRIBUTING.md that covers:

1. A brief overview of table-driven development (with a link to a great intro resource)
2. A few rules of thumb for test-writing in the Osmosis repo
3. Three examples/templates (a message-related test, a keeper-related test, and a gamm-related test) with brief intros, permalinks, and code blocks for people to copy-paste from as needed

Testing and Verifying

This change is a trivial rework / code cleanup without any test coverage.

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes? (no)
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? (no)
• How is the feature or change documented? (not documented)

[xBalbinus]

Lgtm! Great additions on testing!

p.s. be sure to fix the markdown lint errors :)

[stackman27]

Thoughts on making the examples collapsible, something like this:

Example #1: [Message-Related Test]

func (suite *KeeperTestSuite) TestBurnDenom() {
  var addr0bal int64
  // Create a denom.
  suite.CreateDefaultDenom()
  // mint 10 default token for testAcc[0]
  suite.msgServer.Mint(sdk.WrapSDKContext(suite.Ctx), types.NewMsgMint(suite.TestAccs[0].String(), sdk.NewInt64Coin(suite.defaultDenom, 10)))
  addr0bal += 10
  for _, tc := range []struct {
  	desc      string
  	amount    int64
  	burnDenom string
  	admin     string
  	valid     bool
  }{
  	{
  		desc:      "denom does not exist",
  		amount:    10,
  		burnDenom: "factory/osmo1t7egva48prqmzl59x5ngv4zx0dtrwewc9m7z44/evmos",
  		admin:     suite.TestAccs[0].String(),
  		valid:     false,
  	},
  	{
  		desc:      "burn is not by the admin",
  		amount:    10,
  		burnDenom: suite.defaultDenom,
  		admin:     suite.TestAccs[1].String(),
  		valid:     false,
  	},
  	{
  		desc:      "burn amount is bigger than minted amount",
  		amount:    1000,
  		burnDenom: suite.defaultDenom,
  		admin:     suite.TestAccs[1].String(),
  		valid:     false,
  	},
  	{
  		desc:      "success case",
  		amount:    10,
  		burnDenom: suite.defaultDenom,
  		admin:     suite.TestAccs[0].String(),
  		valid:     true,
  	},
  } {
  	suite.Run(fmt.Sprintf("Case %s", tc.desc), func() {
  		// Test minting to admins own account
  		_, err := suite.msgServer.Burn(sdk.WrapSDKContext(suite.Ctx), types.NewMsgBurn(tc.admin, sdk.NewInt64Coin(tc.burnDenom, 10)))
  		if tc.valid {
  			addr0bal -= 10
  			suite.Require().NoError(err)
  			suite.Require().True(suite.App.BankKeeper.GetBalance(suite.Ctx, suite.TestAccs[0], suite.defaultDenom).Amount.Int64() == addr0bal, suite.App.BankKeeper.GetBalance(suite.Ctx, suite.TestAccs[0], suite.defaultDenom))
  		} else {
  			suite.Require().Error(err)
  			suite.Require().True(suite.App.BankKeeper.GetBalance(suite.Ctx, suite.TestAccs[0], suite.defaultDenom).Amount.Int64() == addr0bal, suite.App.BankKeeper.GetBalance(suite.Ctx, suite.TestAccs[0], suite.defaultDenom))
  		}
  	})
  }
}

[p0mvn]

Awesome work on this. I really like the examples at multiple levels of abstraction

[p0mvn]

nit: might be just my personal view so please feel free to ignore it if you disagree but I feel like having a link is sufficient.

If there is a link, I usually skip code blocks in READMEs

[AlpinYukseloglu]

Since our goal is to make it as straightforward (and fast) as possible to copy-paste test templates, I personally prefer to keep the code blocks in there. This also lets us update/change/move the tests being linked to without having to worry about keeping the templates simple and relevant for copy-pasting