gdb/testsuite: make gdb.ada/mi_prot.exp stop at expected location

gdb/testsuite: make gdb.ada/mi_prot.exp stop at expected location

This test attempts to run until the line marked "STOP", which is at
prot.adb:34.  It first runs until the "main" symbol, then tries to place
a breakpoint by line at line 34, without specifying the source file.  When looking at the logs:

    -break-insert -t 34^M
    ^done,bkpt={number="2",type="breakpoint",disp="del",enabled="y",addr="0x0000555555558a6c",func="adafinal",file="/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.ada/mi_pro    t/b~prot.adb",fullname="/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.ada/mi_prot/b~prot.adb",line="44",thread-groups=["i1"],times="0",original-location="/home/simark/b    uild/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.ada/mi_prot/b~prot.adb:34"}^M
    ... continues ...
     *stopped,reason="breakpoint-hit",disp="del",bkptno="2",frame={addr="0x0000555555558a6c",func="adafinal",args=[],file="/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.ada/    mi_prot/b~prot.adb",fullname="/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.ada/mi_prot/b~prot.adb",line="44",arch="i386:x86-64"},thread-id="1",stopped-threads="all",co    re="8"^M

... we see that the breakpoint is placed in some generated file, not in
the test source file as we expect.  The problem is that "b main" in Ada
does not place a breakpoint on the "Ada main", but on some symbol in a
generated source file.  So when stopped at the "main" symbol, we are not
stopped in the file that contains the STOP marker at line 34.

The test passes anyway today, so it doesn't seem to matter that we are
stopped at an unexpected location.  But it starts failing with this
patch [1], because b~prot.adb:34 happens to be between two functions, so
the breakpoint doesn't resolve.

Fix this by placing the breakpoint at "$srcfile:$line", which works
regardless of what is the current source file.

However, this ends up introducing a path in the test name.  Modify
mi_tbreak and mi_continue_to_line to avoid that.

[1] https://sourceware.org/pipermail/gdb-patches/2022-April/187686.html

Change-Id: I742e2a9993046dcb5e30c64fe2ad920a363baf75