Replace calls to deprecated `com.google.errorprone.CompilationTestHelper#addSourceFile` in tests

[msridhar]

E.g., here:

NullAway/nullaway/src/test/java/com/uber/nullaway/UnannotatedTests.java

Line 12 in f1c302e

.addSourceFile("testdata/Shape_Stuff.java")

We should call com.google.errorprone.CompilationTestHelper#addSourceLines instead and copy the relevant source into a text block, and then delete the separate source files. We can also remove any related @SuppressWarnings("deprecation") annotations.

[msridhar]

#1426 has landed, which addresses this partially, but we still need to address all the addSourceFile calls in other tests.