tests: Add full feature and coverage tests for PEP 750 template strings.

Includes corresponding .exp files because this feature is only available in
Python 3.14+.

Tests for `!a` conversion specifier and space after `!` are not included
because they are not supported by MicroPython.

Signed-off-by: Koudai Aono <koxudaxi@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>

tests/basics/string_tstring_constructor.py

@@ -0,0 +1,112 @@
+from string.templatelib import Template, Interpolation
+
+print("=== Constructor basic usage ===")
+t = Template("hello ", Interpolation(42, "x"), "world")
+print(f"Template repr: {repr(t)}")
+
+t_varargs = Template("Hello ", Interpolation("World", "name"), "!")
+print(f"Varargs constructor: strings={t_varargs.strings}, values={t_varargs.values}")
+
+t_concat = Template("A", "B", Interpolation(1, "value"), "C", "D")
+print(f"Varargs merged strings: {t_concat.strings}")
+
+t_leading = Template(Interpolation(1, "x"), " tail")
+print(f"Leading interpolation strings: {t_leading.strings}")
+
+t_trailing = Template("head ", Interpolation(2, "y"))
+print(f"Trailing interpolation strings: {t_trailing.strings}")
+
+t_interps_only = Template(Interpolation(1, "x"), Interpolation(2, "y"))
+print(f"Interpolation only strings: {t_interps_only.strings}")
+
+print("\n=== Special cases ===")
+
+i = Interpolation(42, "x", "s", ":>10")
+try:
+    i.value = 100
+except AttributeError:
+    print("Interp read-only: AttributeError")
+
+t_ws_trim = Template("", Interpolation(None, "   ", None, ""), "")
+print(f"Whitespace trim: '{t_ws_trim}'")
+
+t_debug = Template("", Interpolation(42, "x=", None, ""), "")
+print(f"Debug =: {t_debug}")
+
+
+class Custom:
+    def __repr__(self):
+        return "CustomRepr"
+
+    def __str__(self):
+        return "CustomStr"
+
+
+obj = Custom()
+print(f"Custom !r: {t'{obj!r}'}")
+print(f"Custom !s: {t'{obj!s}'}")
+
+t_empty_start = Template("", Interpolation(1, "1"), "text")
+print(f"Empty start iter: {[type(x).__name__ for x in t_empty_start]}")
+
+t_iter_edge = Template("", Interpolation(1, "1"), "", Interpolation(2, "2"), "")
+iter_items = []
+for item in t_iter_edge:
+    iter_items.append(type(item).__name__)
+print(f"Iterator edge: {iter_items}")
+
+print("\n=== Values property ===")
+for n in range(7):
+    args = []
+    for i in range(n):
+        args.append("")
+        args.append(Interpolation(i, str(i)))
+    args.append("")
+    t = Template(*args)
+    print(f"Values[{n}]: {t.values}")
+
+print("\n=== Multiple consecutive strings ===")
+try:
+    t = Template("first", "second", "third", Interpolation(42, "x"), "fourth", "fifth")
+    if t.strings == ("firstsecondthird", "fourthfifth"):
+        print("Multiple strings concatenated: OK")
+    else:
+        print(f"Multiple strings: strings={t.strings}")
+except Exception as e:
+    print(f"Multiple strings error: {e}")
+
+print("\n=== Template() constructor with many interpolations ===")
+try:
+    exprs = ["a", "b", "c", "d", "e"]
+    interps = [Interpolation(i, exprs[i % len(exprs)]) for i in range(20)]
+    strings = [""] * 21
+    t = Template(*strings, *interps)
+    print(f"Template() constructor: OK ({len(t.interpolations)} interpolations)")
+except Exception as e:
+    print(f"Template() constructor: {type(e).__name__}")
+
+print("\n=== vstr string concatenation ===")
+try:
+    t1 = Template("part1", "part2", "part3", "part4", Interpolation(1, "x"), "end")
+    result = str(t1)
+    print(f"vstr concat: '{result}'")
+except Exception as e:
+    print(f"vstr concat error: {e}")
+
+print("\n=== High byte handling ===")
+try:
+    result = t"\x7f\x80\x81\xfe\xff"
+    first_str = result.strings[0]
+    print(
+        f"High bytes: len={len(first_str)}, first=0x{ord(first_str[0]):02x}, last=0x{ord(first_str[-1]):02x}"
+    )
+except Exception as e:
+    print(f"High bytes error: {e}")
+
+try:
+    result = t"\200\201\377"
+    print(f"Octal high bytes: OK, len={len(result.strings[0])}")
+except Exception as e:
+    print(f"Octal high bytes error: {e}")
+
+print("\nConstructor tests completed!")