feat: add default value for var

[guoweis-work]

today, if one of the json elements can't be found, none of the resolution works

name: Exec testsuite
testcases:
  - name: catjson
    steps:
      - script: cat exec/testa.json
        info: "the value of result.systemoutjson is {{.result.systemoutjson}}"
        assertions:
          - result.systemoutjson.foo ShouldContainSubstring bar
        vars:
          foo:
            from: result.systemoutjson.foo
          bar:
            from: result.systemoutjson.bar
            default: "test"
      - script: echo "{{.catjson.foo}}"
        info: "the value of foo is {{.catjson.foo}} {{.catjson.bar}}"

this is the output

 • Exec testsuite (tests/exec.yml)
 	• catjson FAILURE
	  [info] the value of result.systemoutjson is {"foo":"bar"} (tests/exec.yml:29)
	  [info] the value of foo is {{.catjson.foo}} {{.catjson.bar}} (tests/exec.yml:39)

This change allows a default value to be specified such that if it's missing from the json, the default value will be used.

[yesnault]

Hi @guoweis-outreach,
Thank you for this proposal. It's ok to include that into venom. Can you:

• add some tests into tests/ directory
• add some doc about this into the README

Thank you

[guoweis-work]

@yesnault does this seem good to you?

[yesnault]

Thank you @guoweis-outreach !

LGTM