Default `flow test` behaviour to test `**/*_test.cdc`

[jribbink]

Closes #1378

Description

Removes mandatory requirement for specifying test file names and defaults to **/*_test.cdc (not a breaking change).

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For contributor use:

• [x] Targeted PR against master branch
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [x] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.00000% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 39.39%. Comparing base (4633ff0) to head (f1a197b).

Files	Patch %	Lines
internal/test/test.go	20.00%	22 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1550 +/- ## ========================================== - Coverage 39.66% 39.39% -0.28% ========================================== Files 51 51 Lines 3300 3323 +23 ========================================== Hits 1309 1309 - Misses 1826 1847 +21 - Partials 165 167 +2 ``` | [Flag](https://app.codecov.io/gh/onflow/flow-cli/pull/1550/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=onflow) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/onflow/flow-cli/pull/1550/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=onflow) | `39.39% <20.00%> (-0.28%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=onflow#carryforward-flags-in-the-pull-request-comment) to find out more.

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