Exit with non-0 return code if no tests were run

A new flag, `--allow-running-no-tests` was added to override this
behaviour if exit code of 0 was desired.

This change also made `-w NoTests` obsolete, and so it has been
removed.

tests/SelfTest/WarnAboutNoTests.cmake

@@ -1,19 +0,0 @@
-# Workaround for a peculiarity where CTest disregards the return code from a
-# test command if a PASS_REGULAR_EXPRESSION is also set
-execute_process(
-    COMMAND ${CMAKE_ARGV3} -w NoTests "___nonexistent_test___"
-    RESULT_VARIABLE ret
-    OUTPUT_VARIABLE out
-)
-
-message("${out}")
-
-if(NOT ${ret} MATCHES "^[0-9]+$")
-    message(FATAL_ERROR "${ret}")
-endif()
-
-if(${ret} EQUAL 0)
-    message(FATAL_ERROR "Expected nonzero return code")
-elseif(${out} MATCHES "Helper failed with")
-    message(FATAL_ERROR "Helper failed")
-endif()