Allow to test a custom fixtures path during testing

Of course, these won't really be fixtures, but it allows to test against whole codebases
without copying them, doing symlinks or something like that.

For example, I can tell that over the whole RuboCop codebase, there are only 8 files that produce mismatched ast.
Telling what the problem is is a different problem. The ast for real files can and will be huge so I haven't checked yet
(maybe parser bug) but it's nice for discoverability regardless

Author: Earlopain

test/prism/ruby/parser_test.rb

@@ -167,7 +167,7 @@ def assert_equal_parses(fixture, compare_asts: true, compare_tokens: true, compa
         ignore_warnings { Prism::Translation::Parser33.new.tokenize(buffer) }
 
       if expected_ast == actual_ast
-        if !compare_asts
+        if !compare_asts && !Fixture.custom_base_path?
           puts "#{fixture.path} is now passing"
         end
 
@@ -178,7 +178,7 @@ def assert_equal_parses(fixture, compare_asts: true, compare_tokens: true, compa
         rescue Test::Unit::AssertionFailedError
           raise if compare_tokens
         else
-          puts "#{fixture.path} is now passing" if !compare_tokens
+          puts "#{fixture.path} is now passing" if !compare_tokens && !Fixture.custom_base_path?
         end
 
         assert_equal_comments(expected_comments, actual_comments) if compare_comments

test/prism/snapshots_test.rb

@@ -60,7 +60,7 @@ def assert_snapshot(fixture)
         # If the snapshot file exists, then assert that the printed value
         # matches the snapshot.
         assert_equal(saved, printed)
-      else
+      elsif !Fixture.custom_base_path?
         # If the snapshot file does not yet exist, then write it out now.
         directory = File.dirname(snapshot)
         FileUtils.mkdir_p(directory) unless File.directory?(directory)

test/prism/test_helper.rb

@@ -38,7 +38,7 @@ class TestCase < ::Test::Unit::TestCase
     # are used to define test methods that assert against each fixture in some
     # way.
     class Fixture
-      BASE = File.join(__dir__, "fixtures")
+      BASE = ENV.fetch("FIXTURE_BASE", File.join(__dir__, "fixtures"))
 
       attr_reader :path
 
@@ -63,9 +63,14 @@ def test_name
       end
 
       def self.each(except: [], &block)
-        paths = Dir[ENV.fetch("FOCUS") { File.join("**", "*.txt") }, base: BASE] - except
+        glob_pattern = ENV.fetch("FOCUS") { custom_base_path? ? File.join("**", "*.rb") : File.join("**", "*.txt") }
+        paths = Dir[glob_pattern, base: BASE] - except
         paths.each { |path| yield Fixture.new(path) }
       end
+
+      def self.custom_base_path?
+        ENV.key?("FIXTURE_BASE")
+      end
     end
 
     # Yield each encoding that we want to test, along with a range of the