[flang] Update output format test to use GTest

Better document each test in output formatting tests. Use GTest primitives and infrastructure in same
spirit as [[ https://reviews.llvm.org/D97403 | D97403 ]]. [[ flang-compiler/f18#995 (comment) | See legacy github issue linked here ]] for additional context. Reorganize long test cases to be more readable.

Reviewed By: awarzynski, klausler

Differential Revision: https://reviews.llvm.org/D98303

Author: ashermancinelli

Diff for: flang/unittests/Runtime/CMakeLists.txt

@@ -23,11 +23,6 @@ add_flang_nongtest_unittest(format
   FortranRuntime
 )
 
-add_flang_nongtest_unittest(hello
-  RuntimeTesting
-  FortranRuntime
-)
-
 # This test is not run by default as it requires input.
 add_executable(external-hello-world
   external-hello.cpp