base-tests: use subshells to manage current directory

It saves on having to pair `cd <path> && think && cd ..`.

ci/base-tests.sh

@@ -11,14 +11,15 @@ cargo build --features debugging
 cargo test --features debugging
 # for faster build, share target dir between subcrates
 export CARGO_TARGET_DIR=`pwd`/target/
-cd clippy_lints && cargo test && cd ..
-cd rustc_tools_util && cargo test && cd ..
-cd clippy_dev && cargo test && cd ..
+(cd clippy_lints && cargo test)
+(cd rustc_tools_util && cargo test)
+(cd clippy_dev && cargo test)
 
 # make sure clippy can be called via ./path/to/cargo-clippy
-cd clippy_workspace_tests
-../target/debug/cargo-clippy
-cd ..
+(
+  cd clippy_workspace_tests
+  ../target/debug/cargo-clippy
+)
 
 # Perform various checks for lint registration
 ./util/dev update_lints --check