libconfig: add ptest support

libconfig: add ptest support

Add ptest support to enable automated testing of libconfig
using ptest-runner.

The implementation uses libtool --mode=install to properly
install test binaries, avoiding issues with libtool wrapper
scripts. Tests are built via 'make check TESTS=' to compile
without running during the build phase.

The test suite includes 16 tests covering parsing, formatting,
binary/hex values, escaped strings, and various edge cases.
All tests pass successfully on qemux86-64 :
START: ptest-runner
BEGIN: /usr/lib/libconfig/ptest
[TEST] ParsingAndFormatting
parsing testdata/input_0.cfg
parsing testdata/input_1.cfg
parsing testdata/input_2.cfg
parsing testdata/input_3.cfg
parsing testdata/input_4.cfg
parsing testdata/input_5.cfg
parsing testdata/input_6.cfg
parsing testdata/input_7.cfg
[ OK ] ParsingAndFormatting

[TEST] ParseInvalidFiles
[ OK ] ParseInvalidFiles

[TEST] ParseInvalidStrings
[ OK ] ParseInvalidStrings

[TEST] BigInt1
[ OK ] BigInt1

[TEST] BigInt2
[ OK ] BigInt2

[TEST] BigInt3
[ OK ] BigInt3

[TEST] BigInt4
[ OK ] BigInt4

[TEST] BigInt5
[ OK ] BigInt5

[TEST] BigInt6
[ OK ] BigInt6

[TEST] BigInt7
[ OK ] BigInt7

[TEST] RemoveSetting
[ OK ] RemoveSetting

[TEST] EscapedStrings
[ OK ] EscapedStrings

[TEST] OverrideSetting
[ OK ] OverrideSetting

[TEST] SettingLookups
[ OK ] SettingLookups

[TEST] ReadStream
[ OK ] ReadStream

[TEST] BinaryAndHex
some auto big hex: 4294967296
some auto big bin: 8589934591
negativehex: -1430532899
[ OK ] BinaryAndHex

16 tests; 16 passed, 0 failed
PASS: libconfig_tests
DURATION: 0
END: /usr/lib/libconfig/ptest
STOP: ptest-runner
TOTAL: 1 FAIL: 0

Signed-off-by: Pratik Farkase <pratik.farkase@est.tech>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>