Add ability for ponytest to exclude tests based on name

Adds --exclude command line option that can be used to
exclude any tests whose name starts with the given prefix.

As part of this change, I think it makes sense to introduce
a breaking change to rename the "filter" option. It's currently
use to only run those tests that match the filter argument.
Based on the existence of "exclude", I think it makes sense to
change "filter" to "only". "Filter" is very generic. The "exclude"
option is itself a filter.

I think the options are:

1) I amend this commit to rename filter.
2) We merge this as is and do another commit to rename filter
3) We merge this as is and put the filter rename through an RFC

Author: SeanTAllen

packages/ponytest/pony_test.pony

@@ -75,6 +75,12 @@ Arbitrary strings can be used for these names, but for large projects it is
 strongly recommended to use a hierarchical naming scheme to make it easier to
 select groups of tests.
 
+You can skip any tests whose names start with a given string by using the
+`--exclude=[prefix]` command line option.
+
+You can run only tests whose names start with a given string by using the
+`--filter=[prefix]` command line option.
+
 ## Aggregation
 
 Often it is desirable to run a collection of unit tests from multiple different
@@ -207,7 +213,6 @@ actor PonyTest
   let _env: Env
   let _timers: Timers = Timers
   var _do_nothing: Bool = false
-  var _filter: String = ""
   var _verbose: Bool = false
   var _sequential: Bool = false
   var _no_prog: Bool = false
@@ -216,6 +221,10 @@ actor PonyTest
   var _finished: USize = 0
   var _any_found: Bool = false
   var _all_started: Bool = false
+
+  // Filtering options
+  var _exclude: String = ""
+  var _filter: String = ""
   var _label: String = ""
 
   new create(env: Env, list: TestList tag) =>
@@ -231,14 +240,19 @@ actor PonyTest
 
   be apply(test: UnitTest iso) =>
     """
-    Run the given test, subject to our filter and options.
+    Run the given test, subject to our filters and options.
     """
     if _do_nothing then
       return
     end
 
     var name = test.name()
 
+    // Ignore any tests that satisfy our "exclude" filter
+    if name.at(_exclude, 0) then
+      return
+    end
+
     // Ignore any tests that don't satisfy our filter
     if not name.at(_filter, 0) then
       return
@@ -337,7 +351,7 @@ actor PonyTest
     end
 
     if not _any_found then
-      // No tests matched our filter, print special message.
+      // No tests left after applying our filters
       _env.out.print("No tests found")
       return
     end
@@ -376,6 +390,8 @@ actor PonyTest
         _no_prog = true
       elseif arg == "--list" then
         _list_only = true
+      elseif arg.compare_sub("--exclude=", 10) is Equal then
+        _filter = arg.substring(9)
       elseif arg.compare_sub("--filter=", 9) is Equal then
         _filter = arg.substring(9)
       elseif arg.compare_sub("--label=", 8) is Equal then
@@ -387,6 +403,8 @@ actor PonyTest
         _env.out.print("  " + exe_name + " [options]")
         _env.out.print("")
         _env.out.print("Options:")
+        _env.out.print("  --exclude=prefix  - Don't run tests whose names " +
+          "start with the given prefix.")
         _env.out.print("  --filter=prefix   - Only run tests whose names " +
           "start with the given prefix.")
         _env.out.print("  --verbose         - Show all test output.")