test-lib.sh: make $GIT_BUILD_DIR an absolute path

Change the GIT_BUILD_DIR from a path like "/path/to/build/t/.." to
"/path/to/build". The "TEST_DIRECTORY" here is already made an
absolute path a few lines above this.

This will be helpful to LSAN_OPTIONS which will want to strip the
build directory path from filenames, which we couldn't do if we had a
"/.." in there.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>

Author: avar

t/test-lib.sh

@@ -34,7 +34,7 @@ then
 	# elsewhere
 	TEST_OUTPUT_DIRECTORY=$TEST_DIRECTORY
 fi
-GIT_BUILD_DIR="$TEST_DIRECTORY"/..
+GIT_BUILD_DIR="${TEST_DIRECTORY%/t}"
 
 # Prepend a string to a VAR using an arbitrary ":" delimiter, not
 # adding the delimiter if VAR is empty. I.e. a generalized: