encode test fails if HEVC encoder and decoder are built as plugins

[rathann]

If libheif is configured with

 -DWITH_LIBDE265_PLUGIN=ON \
 -DWITH_X265_PLUGIN=ON \

the encode test fails:

/usr/bin/ctest --test-dir redhat-linux-build --output-on-failure --force-new-ctest-process -j4
Internal ctest changing into directory: /builddir/build/BUILD/libheif-1.16.2/redhat-linux-build
Test project /builddir/build/BUILD/libheif-1.16.2/redhat-linux-build
    Start 1: conversion
    Start 2: encode
    Start 3: uncompressed_decode
1/3 Test #2: encode ...........................***Failed    0.01 sec

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
encode is a Catch v2.13.9 host application.
Run with -? for options

-------------------------------------------------------------------------------
ispe odd size
-------------------------------------------------------------------------------
/builddir/build/BUILD/libheif-1.16.2/tests/encode.cc:149
...............................................................................

/builddir/build/BUILD/libheif-1.16.2/tests/encode.cc:124: FAILED:
  REQUIRE( err.code == heif_error_Ok )
with expansion:
  3 == 0

===============================================================================
test cases: 1 | 1 failed
assertions: 1 | 1 failed


2/3 Test #3: uncompressed_decode ..............   Passed    0.01 sec
3/3 Test #1: conversion .......................   Passed    0.13 sec

67% tests passed, 1 tests failed out of 3

Total Test time (real) =   0.14 sec

The following tests FAILED:
	  2 - encode (Failed)
Errors while running CTest

Could the tests be fixed to work even if the encoders and decoders are built as plugins?

[farindk]

See #986.
When building the tests with plugins, you probably have to set the LIBHEIF_PLUGIN_PATH to the plugin files location since they are not installed in the system without make install.