Add "fail-blank" config option

src/spectator/command_line_arguments_config_source.cr

@@ -15,6 +15,7 @@ module Spectator
       OptionParser.parse(@args) do |parser|
         parser.on("-v", "--verbose", "Verbose output using document formatter") { builder.formatter = Formatting::DocumentFormatter.new }
         parser.on("-f", "--fail-fast", "Stop testing on first failure") { builder.fail_fast }
+        parser.on("-b", "--fail-blank", "Fail if there are no examples") { builder.fail_blank }
         parser.on("-h", "--help", "Show this help") { puts parser; exit }
         parser.on("-e", "--example STRING", "Run examples whose full nested names include STRING") { |pattern| raise NotImplementedError.new("-e") }
         parser.on("-l", "--line LINE", "Run examples whose line matches LINE") { |line| raise NotImplementedError.new("-l") }

src/spectator/config.cr

@@ -7,10 +7,14 @@ module Spectator
     # Indicates whether the test should abort on first failure.
     getter? fail_fast : Bool
 
+    # Indicates whether the test should fail if there are no examples.
+    getter? fail_blank : Bool
+
     # Creates a new configuration.
     def initialize(builder)
       @formatter = builder.formatter
       @fail_fast = builder.fail_fast?
+      @fail_blank = builder.fail_blank?
     end
   end
 end

src/spectator/config_builder.cr

@@ -10,6 +10,7 @@ module Spectator
 
     @formatter : Formatting::Formatter? = nil
     @fail_fast = false
+    @fail_blank = false
 
     # Sets the formatter to use for reporting test progress and results.
     def formatter=(formatter : Formatting::Formatter)
@@ -44,6 +45,22 @@ module Spectator
       @fail_fast
     end
 
+    # Enables fail-blank mode (fail on no tests).
+    def fail_blank
+      self.fail_blank = true
+    end
+
+    # Sets teh fail-blank mode.
+    def fail_blank=(flag)
+      @fail_blank = flag
+    end
+
+    # Indicates whether fail-fast mode is enabled.
+    # That is, it is a failure if there are no tests.
+    def fail_blank?
+      @fail_blank
+    end
+
     # Creates a configuration.
     def build : Config
       Config.new(self)