make the build runner and test runner talk to each other

std.Build.addTest creates a CompileStep as before, however, this kind of
step no longer actually runs the unit tests. Instead it only compiles
it, and one must additionally create a RunStep from the CompileStep in
order to actually run the tests.

RunStep gains integration with the default test runner, which now
supports the standard --listen=- argument in order to communicate over
stdin and stdout. It also reports test statistics; how many passed,
failed, and leaked, as well as directly associating the relevant stderr
with the particular test name that failed.

This separation of CompileStep and RunStep means that
`CompileStep.Kind.test_exe` is no longer needed, and therefore has been
removed in this commit.

 * build runner: show unit test statistics in build summary
 * added Step.writeManifest since many steps want to treat it as a
   warning and emit the same message if it fails.
 * RunStep: fixed error message that prints the failed command printing
   the original argv and not the adjusted argv in case an interpreter
   was used.
 * RunStep: fixed not passing the command line arguments to the
   interpreter.
 * move src/Server.zig to std.zig.Server so that the default test runner
   can use it.
 * the simpler test runner function which is used by work-in-progress
   backends now no longer prints to stderr, which is necessary in order
   for the build runner to not print the stderr as a warning message.

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