pkg/specgenutilexternal: add tests for FindMountType

[ROKUMATE]

What

added tests for FindMountType parses --mount strings with a table driven coverage for that

Checklist

Ensure you have completed the following checklist for your pull request to be reviewed:

• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all
  commits. (git commit -s). (If needed, use git commit -s --amend). The author email must match
  the sign-off email address. See CONTRIBUTING.md
  for more information.
• Referenced issues using Fixes: #00000 in commit message (if applicable)
• Tests have been added/updated (or no tests are needed)
• Documentation has been updated (or no documentation changes are needed)
• All commits pass make validatepr (format/lint checks)
• Release note entered in the section below (or None if no user-facing changes)

Does this PR introduce a user-facing change?

none

[Honny1]

I would like to see also some other cases for example:

• type=bind,src=\"/path,with,commas\",target=/bar" (Quoting it could be problematic)
• type=,target=/bar"(empty key)
• type = a,target=/bar" (some unexpected white spaces)

Othervise look good.