Add test for InstrumentedEnqueueRequestForObject

[jmrodri]

Added a test for the InstrumentedEnqueueRequestForObject to increase the code coverage.

Before the test, handler was at 71.2%

$ make test                                                          
ok      github.com/operator-framework/operator-lib/handler      1.080s  coverage: 71.2% of statements   
...

After adding the test, coverage goes to 100%

$ make test                                                  
ok      github.com/operator-framework/operator-lib/handler      1.082s  coverage: 100.0% of statements
...

[coveralls]

Pull Request Test Coverage Report for Build 188099145

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+8.8%) to 80.052%

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Totals
Change from base Build 187369912:	8.8%
Covered Lines:	309
Relevant Lines:	386

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💛 - Coveralls

[camilamacedo86]

Just a few nits:

• Could we add the suffix _test which has special meaning for the Go compiler? So, it would be package handler_test instead of package handler
  `

[jmrodri]

Just a few nits:

* Could we add the suffix `_test` which has [special meaning for the Go compiler](https://github.com/golang/go/issues/25223)? So, it would be `package handler_test` instead of `package handler`
  `

No, I wanted it in the same package as the code so I could also test the unexported methods. If we put it in package handler_test then we can only test the exported method which is great but sometimes it's easier to test the error conditions of the unexported methods directly.

[camilamacedo86]

/lgtm 👍

[joelanford]

/lgtm