terminal: fix crash in header reporting when absolute testpaths is used

Regressed in 6.1.0 in 62e249a1f9.
The `x` is an `str` but is expected to be a `pathlib.Path`. Not caught
by mypy because `config.getini()` returns `Any`.

Fix by just removing the `bestrelpath` call:

- testpaths are always relative to the rootdir, it thus would be very
  unusual to specify an absolute path there.

- The code was wrong even before the regression: `py.path.local`'s
  `bestrelpath` function expects a `py.path.local`, not an `str`. But it
  had some weird `try ... except AttributeError` fallback which just
  returns the argument, i.e. it was a no-op. So there is no behavior
  change.

- It seems reasonable to me to just print the full path if that's what
  the ini specifies.

changelog/7814.bugfix.rst

@@ -0,0 +1 @@
+Fixed crash in header reporting when :confval:`testpaths` is used and contains absolute paths (regression in 6.1.0).

src/_pytest/terminal.py

@@ -718,10 +718,10 @@ class TerminalReporter:
         if config.inipath:
             line += ", configfile: " + bestrelpath(config.rootpath, config.inipath)
 
-        testpaths = config.getini("testpaths")
+        testpaths = config.getini("testpaths")  # type: List[str]
         if testpaths and config.args == testpaths:
-            rel_paths = [bestrelpath(config.rootpath, x) for x in testpaths]
-            line += ", testpaths: {}".format(", ".join(rel_paths))
+            line += ", testpaths: {}".format(", ".join(testpaths))
+
         result = [line]
 
         plugininfo = config.pluginmanager.list_plugin_distinfo()

testing/test_terminal.py

@@ -18,6 +18,7 @@ import pytest
 from _pytest._io.wcwidth import wcswidth
 from _pytest.config import Config
 from _pytest.config import ExitCode
+from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pathlib import Path
 from _pytest.pytester import Testdir
 from _pytest.reports import BaseReport
@@ -749,6 +750,29 @@ class TestTerminalFunctional:
         result = testdir.runpytest("tests")
         result.stdout.fnmatch_lines(["rootdir: *test_header0, configfile: tox.ini"])
 
+    def test_header_absolute_testpath(
+        self, testdir: Testdir, monkeypatch: MonkeyPatch
+    ) -> None:
+        """Regresstion test for #7814."""
+        tests = testdir.tmpdir.join("tests")
+        tests.ensure_dir()
+        testdir.makepyprojecttoml(
+            """
+            [tool.pytest.ini_options]
+            testpaths = ['{}']
+        """.format(
+                tests
+            )
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(
+            [
+                "rootdir: *absolute_testpath0, configfile: pyproject.toml, testpaths: {}".format(
+                    tests
+                )
+            ]
+        )
+
     def test_no_header(self, testdir):
         testdir.tmpdir.join("tests").ensure_dir()
         testdir.tmpdir.join("gui").ensure_dir()