Add some basic tests for the `siProp` proofmode.

tests/proofmode_siprop.v

+From iris.proofmode Require Import tactics .
+From iris.si_logic Require Import bi.
+Set Ltac Backtrace.
+
+Section si_logic_tests.
+  Implicit Types P Q R : siProp.
+
+  Lemma test_everything_persistent P : P -∗ P.
+  Proof. by iIntros "#HP". Qed.
+
+  Lemma test_everything_affine P : P -∗ True.
+  Proof. by iIntros "_". Qed.
+
+  Lemma test_iIntro_impl P Q R : (P → Q ∧ R → P ∧ R)%I.
+  Proof. iIntros "#HP #[HQ HR]". auto. Qed.
+
+  Lemma test_iApply_impl_1 P Q R : (P → (P → Q) → Q)%I.
+  Proof. iIntros "HP HPQ". by iApply "HPQ". Qed.
+
+  Lemma test_iApply_impl_2 P Q R : (P → (P → Q) → Q)%I.
+  Proof. iIntros "#HP #HPQ". by iApply "HPQ". Qed.
+End si_logic_tests.