我是Ubuntu的新手，无法安装testthat。
跑步
install.packages('testthat')
产生以下输出：
Installing package into ‘/home/user/R/x86_64-pc-linux-gnu-library/3.5’
(as ‘lib’ is unspecified)
trying URL 'https://cloud.r-project.org/src/contrib/testthat_2.0.1.tar.gz'
Content type 'application/x-gzip' length 217071 bytes (211 KB)
==================================================
downloaded 211 KB
* installing *source* package ‘testthat’ ...
** package ‘testthat’ successfully unpacked and MD5 sums checked
** libs
gcc -std=gnu99 -I"/usr/share/R/include" -DNDEBUG -I../inst/include -DCOMPILING_TESTTHAT -fpic -g -O2 -fdebug-prefix-map=/build/r-base-i39faS/r-base-3.5.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g -c init.c -o init.o
gcc -std=gnu99 -I"/usr/share/R/include" -DNDEBUG -I../inst/include -DCOMPILING_TESTTHAT -fpic -g -O2 -fdebug-prefix-map=/build/r-base-i39faS/r-base-3.5.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g -c reassign.c -o reassign.o
g++ -I"/usr/share/R/include" -DNDEBUG -I../inst/include -DCOMPILING_TESTTHAT -fpic -g -O2 -fdebug-prefix-map=/build/r-base-i39faS/r-base-3.5.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g -c test-catch.cpp -o test-catch.o
g++ -I"/usr/share/R/include" -DNDEBUG -I../inst/include -DCOMPILING_TESTTHAT -fpic -g -O2 -fdebug-prefix-map=/build/r-base-i39faS/r-base-3.5.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g -c test-example.cpp -o test-example.o
g++ -I"/usr/share/R/include" -DNDEBUG -I../inst/include -DCOMPILING_TESTTHAT -fpic -g -O2 -fdebug-prefix-map=/build/r-base-i39faS/r-base-3.5.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g -c test-runner.cpp -o test-runner.o
g++ -shared -L/usr/lib/R/lib -Wl,-Bsymbolic-functions -Wl,-z,relro -o testthat.so init.o reassign.o test-catch.o test-example.o test-runner.o -L/usr/lib/R/lib -lR
installing to /home/user/R/x86_64-pc-linux-gnu-library/3.5/testthat/libs
** R
** inst
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
*** copying figures
** building package indices
** installing vignettes
** testing if installed package can be loaded
* DONE (testthat)
The downloaded source packages are in
‘/tmp/RtmpYhp7aJ/downloaded_packages’
当我尝试使用功能Expect_equal（）时，我遇到了
could not find function "expect_equal"
这是权限问题吗？有解决此问题的安全方法吗？
p.s。请原谅我的无知和礼节，并请告知我适当的规程。