fix(core): repair nesting of flux expressions

[sranka]

Fixes #251

This PR fixes the flux template function so that

• a non-interpolated (simple) string returns a flux expression
• an empty flux expression can be wrapped into another flux expression

It also improves and repairs the tests of the flux template function, some of them were not executed due to incorrect usage of mocha.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• yarn test completes successfully
• Commit messages are in semantic format

[codecov-commenter]

Codecov Report

Merging #252 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #252   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines          963       966    +3     
  Branches       210       212    +2     
=========================================
+ Hits           963       966    +3     

Impacted Files	Coverage Δ
packages/core/src/query/flux.ts	100.00% <100.00%> (ø)

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[backbone87]

lgtm,

have you checked the empty expression error?

const inner = last ? flux`|> last()` : flux``;
const outer = flux`from(bucket: "a") ${last} |> yield()`;