e2fsprogs: Fix error SRCDIR when using usrmerge DISTRO_FEATURES

    When build e2fsprogs ptest with usrmerge DISTRO_FEATURES,
    in test_script and test_one scripts, value of SRCDIR has problem.
      SRCDIR=/usr/usr/lib/e2fsprogs/ptest/test

    ptest log
      # ptest-runner e2fsprogs
      START: ptest-runner
      2022-12-17T11:08
      BEGIN: /usr/lib/e2fsprogs/ptest
      ls: cannot access '/usr/usr/lib/e2fsprogs/ptest/test/[a-zA-Z]_*': No such file or directory
      ./test_script: line 54: /usr/usr/lib/e2fsprogs/ptest/test/test_post: No such file or directory
      DURATION: 0
      END: /usr/lib/e2fsprogs/ptest
      2022-12-17T11:08
      STOP: ptest-runner
      TOTAL: 1 FAIL: 0

    Reason
      In Makefile.in, SRCDIR is set by prefix and libdir.
        @echo "SRCDIR=${prefix}${libdir}/e2fsprogs/ptest/test" >> test_one
        @echo "SRCDIR=${prefix}${libdir}/e2fsprogs/ptest/test" >> test_script
      prefix=/usr
      libdir=/usr/lib (when usrmerge is set)

    solution
      Use PTEST_PATH to fix it.

Signed-off-by: Qiu Tingting <qiutt@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 85e3b4f1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>