Improve GEMINI.md and test.py documentation

Change-Id: Ifbaeb34c313bfb2812f4a28d38a87210f232625d

GEMINI.md

diff --git a/GEMINI.md b/GEMINI.md
index 732d22c..526a5fe 100644
--- a/GEMINI.md
+++ b/GEMINI.md
@@ -7,7 +7,7 @@
 *   **D8:** A dexer that converts Java bytecode to DEX format.
 *   **R8:** A shrinker and minifier that optimizes Java bytecode and converts it to DEX format.
 
-The project is written in Java and Kotlin and uses Gradle for building. It is a multi-module project with several components, including `main`, `library_desugar`, and `test`.
+The Git project is written in Java and Kotlin and uses Gradle for building. It is a multi-module project with several components, including `main`, `library_desugar`, and `test`.
 
 ## Building and Running
 
@@ -21,6 +21,20 @@
 
 This will produce a JAR file at `build/libs/r8.jar`.
 
+For a lighter target that just compiles the code without building the full R8 JAR, you can use:
+
+```bash
+tools/gradle.py classes
+```
+
+### Disabling Error Prone
+
+Compilation is usually pretty strict due to Error Prone. You can turn it off for debugging-style edits (e.g., leaving unused imports) by passing the property `-Pdisable_errorprone`:
+
+```bash
+tools/gradle.py classes -Pdisable_errorprone
+```
+
 ### Running D8
 
 D8 is used to convert Java class files to the DEX format.
@@ -68,6 +82,9 @@
 tools/test.py --no-internal
 ```
 
+> [!NOTE]
+> `test.py` invokes Gradle, so avoid running concurrent invocations of it to prevent conflicts.
+
 By default, this runs tests using r8lib.jar, which is a bootstrapped R8. It is possible to speed up local testing by running tests with a non-bootstrapped R8:
 
 ```bash
@@ -77,7 +94,13 @@
 It is possible to run a single test by passing the name of the test, e.g.,
 
 ```bash
-tools/test.py --no-internal *ProguardConfigurationParserTest*
+tools/test.py --no-internal --no-r8lib *ProguardConfigurationParserTest*
+```
+
+`tools/test.py` can take multiple additive filters.
+
+```bash
+tools/test.py --no-internal --no-r8lib *ProguardConfigurationParserTest* *ClassInlinerTest*
 ```
 
 ## Development Conventions

tools/test.py

diff --git a/tools/test.py b/tools/test.py
index c2b76b1..634f2ee 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -289,6 +289,10 @@
                         help='Only compile project and tests, do not run tests',
                         default=False,
                         action='store_true')
+    result.add_argument(
+        'filter',
+        nargs='*',
+        help='Test filters. They are additive and support wildcards (e.g., *).')
     options, args = result.parse_known_args()
     if options.java_max_memory_size is None:
         # YouTubeV1719Test OOM's with 4G, so raise xmx to 6G when running internal tests.
@@ -359,7 +363,7 @@
 
 def Main():
     (options, args) = ParseOptions()
-    return_code = test(options, args)
+    return_code = test(options, options.filter)
     if return_code == 0 and options.land:
         land(options)
     return return_code