Address LeakSanitizer errors in unit tests

[nirosys]

Issue #, if available: None

Description of changes:
Linux builds with more recent versions of GCC or clang are generating leak sanitizer errors. These errors include the stack traces where an allocation occurred, the number of times those allocations were not freed, and the total resulting size of the leaked data:

Direct leak of 917504 byte(s) in 14 object(s) allocated from:
    #0 0x4e6b2d in malloc (/home/runner/work/ion-c/ion-c/build/debug/test/all_tests+0x4e6b2d)
    #1 0x7fe40a6955ae in _ion_alloc_block /home/runner/work/ion-c/ion-c/ionc/ion_allocation.c:182:41
    #2 0x7fe40a695512 in _ion_alloc_owner /home/runner/work/ion-c/ion-c/ionc/ion_allocation.c:44:17
    #3 0x7fe40a6f446b in _ion_reader_make_new_reader /home/runner/work/ion-c/ion-c/ionc/ion_reader.c:291:29
    #4 0x7fe40a6f3e8e in _ion_reader_open_buffer_helper /home/runner/work/ion-c/ion-c/ionc/ion_reader.c:48:5
    #5 0x7fe40a6f3b47 in ion_reader_open_buffer /home/runner/work/ion-c/ion-c/ionc/ion_reader.c:32:5
    #6 0x7fe40a87f354 in ion_extractor_path_create_from_ion /home/runner/work/ion-c/ion-c/ionc/ion_extractor.c:218:5
    #7 0x8c4466 in IonExtractorSucceedsWhen_NumPathsAtMaximum_Test::TestBody() /home/runner/work/ion-c/ion-c/test/test_ion_extractor.cpp:650:9

Running unit tests on Ubuntu with clang 13 results in 480 stack traces with leaks. This PR addresses the leaks that were a result of leaked data from tests themselves, which accounts for 196 of the total 480 identified stack traces.

I've grouped all of the tests together into one PR since most of the changes are simply adding frees or otherwise 1 line releasing of resources allocated in the test. I'll add commentary where the changes are not as clear.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[nirosys]

Moved big_int_expected to be heap allocated, to make releasing with ion_int_free possible. Otherwise freeing the internal data for an ION_INT requires implementation knowledge, which can change.

[nirosys]

ION_DECIMAL_EXPECT_OVERFLOW re-uses reader, allocating a new struct which leaks the original reader that was initialized in ION_DECIMAL_TEXT_TO_BINARY.

[nirosys]

With the way ion_test_write_all_values_from_binary is written, the destination &result is expected to be a pointer to a pointer that can receive the address of the buffer that the function will allocate. So re-using result here results in the previous result buffer being leaked, and result containing a new address. This occurs again below. I added more pointers & lengths to track and free the individual buffers.

[nirosys]

ion_test_open_file_writer creates a stream that is not freed when the writer is closed. The caller is not given the stream so knowledge of the stream, and how to free it using the writer is needed. I updated the function to return the stream as an argument so that callers will be aware and can easily free it.

[nirosys]

The buffer allocated by ion_string_strdup needs to be freed. The tmp variable will receive the address to the buffer, so it can be freed once the comparison is done, and re-used for each instance.

[tgregg]

Thanks for doing this cleanup!