Add test cases for first two tasks in Salary Calculator

[ghost]

Closes #2573.

Wrote relevant test cases for task 1 and task 2 and removed some useless test cases for task 3 that tested implementation of the previous tasks and not only implementation of the task 3.

Updated the documentation for more clarity and readability and corrected some misspelled words.

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[sanderploegsma]

Thanks for picking this up! I like the changes a lot already, just left a couple of comments to address.

Also, since your contributions to this exercise are non-trivial, feel free to add yourself as a contributor to the exercise's .meta/config.json:

{
  "authors": [
    "TalesDias"
  ],
+ "contributors": [
+   "smicaliz"
+ ],
  "files": {
    "solution": [
      "src/main/java/SalaryCalculator.java"
    ],
    "test": [
      "src/test/java/SalaryCalculatorTest.java"
    ],
    "exemplar": [
      ".meta/src/reference/java/SalaryCalculator.java"
    ],
    "invalidator": [
      "build.gradle"
    ]
  },
  "blurb": "Learn about ternary operators by calculating salaries."
}

[ghost]

Updated! Ready to review.

[sanderploegsma]

LGTM! Thanks for the great work 😄