Document basictest and bootstraptest

Author: chrisseaton

doc/contributor/testing.md

@@ -8,6 +8,8 @@ We talk about several types of tests in TruffleRuby:
 * TCK (`TCK`)
 * Specs (`SPECS`)
 * MRI tests (`MRI`)
+* MRI basictest (`BASIC`)
+* MRI bootstraptest (`BOOT`)
 * C extension tests (`CEXT`)
 * Bundler tests (`BUN`)
 * Compiler tests (`COMP`)
@@ -64,6 +66,23 @@ MRI tests are in `test/mri`.
 
 Run MRI tests with `jt test mri`.
 
+### MRI basictest
+
+MRI's basictest is a smaller set of tests for some basic control structures
+and language features. It is in `test/basictest`.
+
+Run basictest with `jt ruby test/basictest/runner.rb`.
+
+### MRI bootstraptest
+
+MRI's bootstraptest is a smaller set of tests for functionality they require
+to bootstrap their implementation, including some tests against regressions
+and corner cases. It is in `test/bootstraptest`.
+
+Run bootstraptest with `jt ruby test/bootstraptest/runner.rb`. It's not
+tractable to run bootstraptest with the JVM, as it starts a new Ruby process
+for each test. Run it with a native build instead.
+
 ### C extension tests
 
 C extension tests are a basic test of compiling, loading and running C