add some more f-string tests

tests/run/fstring.pyx

+# mode: run
+# tag: f_strings, pep498
+
+####
+# Cython specific PEP 498 tests in addition to test_fstring.pyx from CPython
+####
+
+def format2(ab, cd):
+    """
+    >>> a, b, c = format2(1, 2)
+    >>> print(a)
+    ab2
+    >>> print(b)
+    1cd
+    >>> print(c)
+    12
+
+    >>> a, b, c = format2('ab', 'cd')
+    >>> print(a)
+    abcd
+    >>> print(b)
+    abcd
+    >>> print(c)
+    abcd
+    """
+    a = f"ab{cd}"
+    assert isinstance(a, unicode), type(a)
+    b = f"{ab}cd"
+    assert isinstance(b, unicode), type(b)
+    c = f"{ab}{cd}"
+    assert isinstance(c, unicode), type(c)
+    return a, b, c
+
+
+def format_c_numbers(int n, float f, double d):
+    """
+    >>> s1, s2, s3, s4 = format_c_numbers(12, 2.3456, 3.1415926)
+    >>> print(s1)
+    122.35
+    >>> print(s2)
+    3.14 2.3
+    >>> print(s3)
+      12f
+    >>> print(s4)
+    C 3.14
+
+    """
+    s1 = f"{n}{f:.3}"
+    assert isinstance(s1, unicode), type(s1)
+    s2 = f"{d:.3}{f:4.2}"
+    assert isinstance(s2, unicode), type(s2)
+    s3 = f"{n:-4}f"
+    assert isinstance(s3, unicode), type(s3)
+    s4 = f"{n:X}{d:5.3}"
+    assert isinstance(s4, unicode), type(s4)
+    return s1, s2, s3, s4
+
+
+def format_strings(str s, unicode u):
+    """
+    >>> a, b, c, d = format_strings('abc', b'xyz'.decode('ascii'))
+    >>> print(a)
+    abcxyz
+    >>> print(b)
+    xyzabc
+    >>> print(c)
+    uxyzsabc
+    >>> print(d)
+    sabcuxyz
+    """
+    a = f"{s}{u}"
+    assert isinstance(a, unicode), type(a)
+    b = f"{u}{s}"
+    assert isinstance(b, unicode), type(b)
+    c = f"u{u}s{s}"
+    assert isinstance(c, unicode), type(c)
+    d = f"s{s}u{u}"
+    assert isinstance(d, unicode), type(d)
+    return a, b, c, d
+
+
+def format_str(str s1, str s2):
+    """
+    >>> a, b, c, d = format_str('abc', 'xyz')
+    >>> print(a)
+    abcxyz
+    >>> print(b)
+    xyzabc
+    >>> print(c)
+    uxyzsabc
+    >>> print(d)
+    sabcuxyz
+    """
+    a = f"{s1}{s2}"
+    assert isinstance(a, unicode), type(a)
+    b = f"{s2}{s1}"
+    assert isinstance(b, unicode), type(b)
+    c = f"u{s2}s{s1}"
+    assert isinstance(c, unicode), type(c)
+    d = f"s{s1}u{s2}"
+    assert isinstance(d, unicode), type(d)
+    return a, b, c, d