Enhanced ts-jest cli to fix warning message.

[beenotung]

Summary

Before this patch, when I run jest test with jest.config.js generated
ts-jest config:init, it shows below warning message:

ts-jest[config] (WARN) Unable to find the root of the project where ts-jest has been installed.

This patch addresses the issue #959

Test plan

A test case in src/cli/cli.spec.ts is updated to reflect this changes.

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build 6106

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 93.209%

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Totals
Change from base Build 6104:	0.003%
Covered Lines:	1108
Relevant Lines:	1142

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💛 - Coveralls

[ahnpnl]

I think this doesn’t fix the actually problem. The problem comes from

ts-jest/src/config/config-set.ts

Line 118 in 82a930b

let pkgPath = resolve(tsJestRoot, '..', '..', 'package.json')

I think instead of this fix, you should provide a fallback mechanism for that piece of codes.