[gdb/testsuite, tclint] Fix gdb.tui

[gdb/testsuite, tclint] Fix gdb.tui

Running tclint on the test-cases in gdb.tui shows a few problems:
...
$ ( cd gdb/testsuite/gdb.tui; tclint --ignore line-length *.exp )
compact-source.exp:58:28: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
compact-source.exp:60:27: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
compact-source.exp:68:32: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
empty.exp:68:2: eval received an argument with a substitution, unable to \
  parse its arguments [command-args]
new-layout.exp:84:2: eval received an argument with a substitution, unable \
  to parse its arguments [command-args]
source-search.exp:33:25: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
wrap-line.exp:40:21: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
wrap-line.exp:44:14: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
wrap-line.exp:62:40: expression with substitutions should be enclosed by \
  braces [unbraced-expr]
...

Fix these.

Tested on aarch64-linux.