nimsuggest: fix and re-enable old tests (#16401)

A number of nimsuggest tests were disabled for various reasons, sometimes due
to brittleness. These tests have been fixed where needed and most have are now
enabled -- details below. The updates are meant to provide better regression
coverage for future nimsuggest improvements. To avoid brittleness some tests
were refactored.

Impact:
* test coverage has now increased
* faster execution of the test suite
* tests are less likely to break due to stdlib changes

Re-enabled Test & Test Description:
* `tchk1.nim`: check (chk) via nimsuggest works at end of file
* `tdot4.nim`: prioritize already used completion
* `tinclude.nim`: definition lookup (def) with includes
* `tstrutils.nim` -> `tdef2.nim`: test template definition lookup (def)
* `tsug_regression.nim`: regression test for [nimsuggest #52](https://github.com/nim-lang/nimsuggest/issues/52)
* `ttemplate_highlight.nim`: per the file name
* `twithin_macro_prefix.nim`: suggest within a macro with a prefix

Tests Not Re-Enabled:
* `twithin_macro.nim` still disabled as it doesn't provide a good test signal
* EPC highlight tests remain disabled -- requires out of scope tester changes

Additional Notes:
* todos added in comments for follow-up work

nimsuggest/tests/tdot3.nim

@@ -9,14 +9,14 @@ proc main(f: Foo) =
 # this way, the line numbers more often stay the same
 
 discard """
-!copy dep_v1.nim dep.nim
+!copy fixtures/mdep_v1.nim dep.nim
 $nimsuggest --tester $file
 >sug $1
 sug;;skField;;x;;int;;*dep.nim;;8;;4;;"";;100;;None
 sug;;skField;;y;;int;;*dep.nim;;8;;8;;"";;100;;None
 sug;;skProc;;tdot3.main;;proc (f: Foo);;$file;;5;;5;;"";;100;;None
 
-!copy dep_v2.nim dep.nim
+!copy fixtures/mdep_v2.nim dep.nim
 >mod $path/dep.nim
 >sug $1
 sug;;skField;;x;;int;;*dep.nim;;8;;4;;"";;100;;None