tests: Rename `RUN_AST_TESTS` variable to `AST_COMPARE` (#1563)
tests: Rename `RUN_AST_TESTS` variable to `AST_COMPARE` (#1563)

This makes it match the environment variable, which is consistent with
the other variables.
rattrayalex/prettier,lydell/prettier,existentialism/prettier,lydell/prettier,arijs/prettier-with-tabs,azz/prettier,arijs/prettier-miscellaneous,azz/prettier,Skywalker13/prettier-space-parenthesis,prettier/prettier,Skywalker13/prettier-space-parenthesis,prettier/prettier,prettier/prettier,existentialism/prettier,rattrayalex/prettier,arijs/prettier-miscellaneous,arijs/prettier-miscellaneous,rattrayalex/prettier,lydell/prettier,jlongster/prettier,arijs/prettier-with-tabs,prettier/prettier,existentialism/prettier,existentialism/prettier,lydell/prettier,Skywalker13/prettier-space-parenthesis,azz/prettier,azz/prettier,Skywalker13/prettier-space-parenthesis,arijs/prettier-miscellaneous