Simplify variable substitution tests ⚗️

Each test case for parameter substitution application was being given a
totally separate test case, with the variables being used being declared
in different places across the file. For tektoncd#1639 I came along and wanted
to start adding more tests for workspace substitution and found it hard
to tell where to start so I:

* Combined most of the test cases for param subsitution into one test
  so you can easily see everything that is being tested (none of the
  test cases conflicted with each other and can easily be applied
  together)
* I kept the array param test cases separate cuz they seemd to be
  testing distinct test cases
* The Volume test cases were a bit odd b/c they were trying to make sure
  substitution was _applied_ to volumes, but there is no volume specific
  function so they were calling an internal function and passing in
  dummy values that are not representative of the actual values you'd
  substitute for volumes so instead I folded these test cases into the
  param application test.

Probably the resource application test case should be made quite similar
to the param test but it seemed like some of the resource stuff was
distinct and had to be tested in isolateion (e.g. just outputs, just
inputs, etc.)

Also removed some depreated (and duplicated!) volume tests:
in tektoncd#1311 I removed support for ${} but instead of removing these tests
I just updated them, making them duplicates of the above test cases.