Add the first test

pygmt/tests/baseline/test_timestamp.png.dvc

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+outs:
+- md5: 885766da8a23cdad2a4cfd96fcda9ea0
+  size: 1759
+  path: test_timestamp.png

pygmt/tests/test_timestamp.py

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+"""
+Tests for Figure.timestamp.
+"""
+import pytest
+from pygmt import Figure
+
+
+@pytest.fixture(scope="module", name="faketime")
+def fixture_faketime():
+    """
+    Fake datetime passed to the "timefmt" parameter so that the timestamp is
+    fixed.
+    """
+    return "1970-01-01T00:00:00"
+
+
+@pytest.mark.mpl_image_compare
+def test_timestamp(faketime):
+    fig = Figure()
+    fig.timestamp(timefmt=faketime)
+    return fig