inputflinger_tests: Put `InputMapperTest` in its own file

I would like to be able to put automated tests for the new
`TouchpadEventMapper` in their own file, rather than
InputReader_tests.cpp. To do this I'll need some of the test utilities
in their own files, too.

Bug: 251196347
Test: atest inputflinger_tests
Change-Id: Ie524adb3431e4a0c59efe75be82abddacd676c21
diff --git a/services/inputflinger/tests/TestConstants.h b/services/inputflinger/tests/TestConstants.h
index 8dc9d71..27881f6 100644
--- a/services/inputflinger/tests/TestConstants.h
+++ b/services/inputflinger/tests/TestConstants.h
@@ -27,4 +27,7 @@
 static constexpr nsecs_t ARBITRARY_TIME = 1234;
 static constexpr nsecs_t READ_TIME = 4321;
 
+// Error tolerance for floating point assertions.
+static const float EPSILON = 0.001f;
+
 } // namespace android