testsuite: Improve test in dg-require-python-h

If GCC is tested with a sysroot which doesn't contain a Python
installation (e.g., with a command such as
"make check-gcc-c FLAGS_UNDER_TEST="--sysroot=/some/path"), but there's
a python3-config in $PATH, then the testsuite will pick up the host's
Python.h which can't actually be used:

Executing on host: python3-config --includes    (timeout = 300)
spawn -ignore SIGHUP python3-config --includes
-I/usr/include/python3.10 -I/usr/include/python3.10
Executing on host: /some/sysroot/bin/aarch64-unknown-linux-gnu-gcc --sysroot=/some/sysroot/libc -Wl,-dynamic-linker=/some/sysroot/libc/lib/ld-linux-aarch64.so.1 -Wl,-rpath=/some/sysroot/libc/lib  /some/src/gcc.git/gcc/testsuite/gcc.dg/plugin/cpython-plugin-test-2.c    -fdiagnostics-plain-output  -fplugin=./analyzer_cpython_plugin.so -fanalyzer -I/usr/include/python3.10 -I/usr/include/python3.10 -S -o cpython-plugin-test-2.s    (timeout = 600)
spawn -ignore SIGHUP /some/sysroot/bin/aarch64-unknown-linux-gnu-gcc --sysroot=/some/sysroot/libc -Wl,-dynamic-linker=/some/sysroot/libc/lib/ld-linux-aarch64.so.1 -Wl,-rpath=/some/sysroot/libc/lib /some/src/gcc.git/gcc/testsuite/gcc.dg/plugin/cpython-plugin-test-2.c -fdiagnostics-plain-output -fplugin=./analyzer_cpython_plugin.so -fanalyzer -I/usr/include/python3.10 -I/usr/include/python3.10 -S -o cpython-plugin-test-2.s
In file included from /usr/include/python3.10/Python.h:8,
                 from /some/src/gcc.git/gcc/testsuite/gcc.dg/plugin/cpython-plugin-test-2.c:8:
/usr/include/python3.10/pyconfig.h:9:12: fatal error: aarch64-linux-gnu/python3.10/pyconfig.h: No such file or directory
compilation terminated.
compiler exited with status 1

This problem causes these testsuite failures:

FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 17)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 18)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 21)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 31)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 32)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 35)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 45)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 55)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 63)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 66)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 68)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so  (test for warnings, line 69)
FAIL: gcc.dg/plugin/cpython-plugin-test-2.c -fplugin=./analyzer_cpython_plugin.so (test for excess errors)
Excess errors:
/usr/include/python3.10/pyconfig.h:9:12: fatal error: aarch64-linux-gnu/python3.10/pyconfig.h: No such file or directory
compilation terminated.

So try to compile a test file so that the testcase can be marked as
unsupported instead.

gcc/testsuite/ChangeLog:
	* lib/target-supports.exp (dg-require-python-h): Test
	whether Python.h can really be used.